First off, there are many manufacturers of line, and the quality between their lines varies quite a bit.  The good manufacturers make sure their line is manufactured under strict quality control and has uniform diameter, tensile strength, knot strength, stretch, and abrasion resistance. The lesser known (a.k.a "budget" variety) lines don't have this consistant quality throughout the whole line and are more likely to have weak points that will break under pressure.

When you start to get into the details of fishing line, you'll want to look at the following qualities:

• Breaking strength
• Density
• Diameter
• Visibility / Color
• Flexibility
• Elasticity
• Abrasion Resistance
• Shock Resistance
• Knot Strength
• Shape

Breaking Strength - In the fishing world, a line's breaking strength is often represented by "pound test" that signifies the amount of stress under which the unknotted line will break.  12-pound test will break when you subject it to 12 pounds of stress, 10-pound test will break when you subject it to 10 pounds of stress, and so on.  When selecting the right weight for your line, you should consider the average weight of the fish you're expecting to catch and the likelihood that you'll hook up with other underwater obstacles such as weeds, tree limbs and rocks.  If you're fishing in areas that are likely to have obstacles, you'll need a heavier line to avoid snaps when you're yanking your line out of the muck.  So why not use the heaviest line available to make sure it never breaks?  The reason is that heavier line also has some disadvantages.  It's wider than lighter line, so fish are more likely to see it.  It's heavier, so it will affect the action of the lure.  And it provides more resistance when retrieving your bait, so it takes longer to retrieve a lure.  So, as a general rule, most seasoned fisherman will suggest your chose the lightest line that you can and match it carefully to the type of fish your after, the fishing obstacles you're likely to come across, and the type of lure you're using.

Density - The density of a line will affect the sink rate and responsiveness of the line during casting or retrieve.  This is the property that guides whether or not a line will float, sink, or suspend.  Multistrand lines typically have lower density than single strand lines and will be more prone to floating.

Diameter - This is the width of the line.  In general, you want to find the lowest diamter possible, because this will improve the line's movement through the air (longer casts) and through the water (better lure motion and retrieve).  Smaller diameter line will also have smaller sized knots, so the knots will be less likely to hit the guides on the rod and get damaged.  To determine the diameter of a line, look for a tiny decimal next the line's pound test number. That number tells you the diameter of the line in tenths or hundredths of a millimeter.  The smaller the decimal, the smaller the diameter.

Visibility/Color - Without getting too much into the science of light penetration and shadowing in water, I'll just say that some colors "dissappear" quickly as you get deeper into the water.  Knowing this, the line manufacturers have developed line colors that allow the fisherman to see the line a few feet below the surface, but prevents the fish from seeing it deeper toward the bottom.  Additionally, many of the modern lines also perform some pretty unique fragmentatin of the light to prevent shadowing on the lake bottom or visiblity of a straight line on the lake surface.  To determine the right color line for your situation, select the line that is least visible in the water you'll be fishing. In murky water, use the greens & blues.  In clear water, use the clear.  Stay away from the flourescent line unless you're using a fishing technique that requires you to see the line on or under the water.  Here's a quick rundown of the most common colors:

• Clear/blue fluorescent - In the sun, you can see this above the surface, but it dissapears underwater.  Good for most fishing conditions. 
• Low-visibility - Does some unique light filtration tricks to make the line less visible underwater.  Good to use when clear line isn't doing the trick.
• High-visibility gold - More visible in the water while the sun is out.  Typically used when trolling multiple lines to prevent tangles or in low-light conditions such as dusk or dawn.
• Coffee - Good for muddy, stained waters.
• Moss green - Good for waters with heavy algae or vegitation.

Flexibility - This term refers to the "limpness" of the line.Wire lines are the stiffest and braided lines are the most limp.  The more flexible a line is, the more likely it is to run straight off the spool without showing the telephone cord loops that stiffer lines are known for having.  If you're setting a hook, you don't want to have a whole lot of extra loops in your line, so flexible line is better for most fishing conditions.  That said, flexible line is also more likely to get snarled and can be tricky on reels prone to birdsnests.  For most lure makers, you'll want to use flexible lines because they will allow your lure to have maximum action.

Elasticity - This term describes how much a line can stretch.  With a few of the high-end "no stretch" types of line, pretty much every line has some amount of stretch.  Polysynthetic / braided microfilament lines have the least elasticity and monofilaments and braided nylon have the most.  The elasticity of a line really comes into play when you've got something on the other end, as elastic lines will be less likely to break under stress and lure movement will appear more realistic to fish.  That said, stretch line does have some disadvantages. It will get weaker with age than non-elastic lines, it may make it harder to feel a strike, and it is more likely to have knots that slip. So, despite the current marketing push for "no stretch" lines, I would suggest you actually keep some of that stretch in your line as it is critical to most fishing techniques.

Abrasion Resistance - This describes a line's ability to withstand being banged and bumped around underwater and in a fish's thrashing grip.  As general rule, multi-stranded lines are more likely to get damaged by wear and tear than single-stranded lines.  Generally, harder lines at higher pound tests that are less elastic are more abrasion resistant than softer, lower pount test, more elastic lines.  For the most part, this quality will let you know how often you'll have to replace your line.  Aside from that, it doesn't really have a big impact on the fishing itself (assuming you replace the line...if you don't then you're more likely to get breaks!).

Shock Resistance - This describes the line's ability to handle a significant and sudden jerk (like a big strike).  If a big sudden jerk occurs to the line, it is more likely to break than if the pull is slow and steady (anyone who's ever yanked on a line that is stuck on an underwater tree limb to break it has seen this property in action).  In general, lines that have more elasticity will be more shock resistant than non-elastic lines.  This property is important if you're fishing for a species that hits hard and fast, or if you're fishing deep waters where you'll need to break the line if you catch a snag.

Knot Strength - Whenever you put a knot in a line, the line itself becomes weaker because the line can cut into itself or unwrap under pressure.  There's really not a whole lot you can do here except understand the specific types of knots that work best with the line you're using.  Matching the right knot to your line will maximize the strength and minimize the chances of snapping or unraveling at the knot.

Shape - Most line looks like a circle when you view it head on, but there are some lines that are actually flat when you see them from the front.  The flat lines have advantages of causing fewer backlashes and allowing more line to sit on a spool.  Aside from those advantages, the shape doesn't appear to have much addiitonal influence on the fishing experience.

So, now that you know all the properties of line, let's look at the different types of line that are out there.  The most basic distinction is monofilament ("mono")  versus braided, and the biggest difference you'll see between the two is that one floats (braided) and the other (mono) sinks.  After you get past that initial shock, you'll notice some other big differences between them.

For starters, braided lines typically have low or no elasticity in them, have higher breaking points than equivalent test mono, have stronger hook-sets, and are more abrasion resistant.  This makes them popular with the tournament anglers who need to quickly muscle lunkers out of the weeds and up onto the boat without the fear of a snapped line.  They also like it because fish manuevers that would typically break mono (like swimming around a dock post) will have no impact on the braided lines.

Those same qualities that make them so powerful can also make them damaging to your equipment.  Braided lines are known for cutting through many types of reel spools and rod guides (and caught fingers!) with ease, so you need to make sure your rod and reel is specifically designed to work with braided lines, and that you keep your fingers out of the path of fast moving line!

Braided lines also have less "memory" and tend to come off the spool without any extra loops in the line and without much wind resistance, making it easier to cast them farther.  This lack of memory also makes it easier to feel a strike immediately and bully a lure through weeds with ease.

So what's the downside?  There are actually a few negatives to braided line aside from the damage it can cause to your equipment.  The biggest drawback is visibility.  Braided line is much more visible to fish than mono, which blends in better with the water.  It also has a quicker retrieve that doesn't work well for all fishing applications, particularly crankbait fishing which needs slower retrieves and "drift back" pauses casued by elastic line (see TackleMaking's image below that shows the impact that line elasticy can have on a crankbait's action underwater).

Aside from Mono and Braided, the other two big categories of line are "Fusion" and "Fluorocarbons".  Fusion is essentially a braided line where the strands are glued together and then coated.  Fluorocarbon is like "super mono".  It comes closes to the refractive index of water so it is virtually invisible to fish.  It doesn't absorb water (mono does), so it maintains its stregth better than mono.  And Fluorocarbon line has low stretch like a braided line, but doesn't float, so it allows lures to run deeper than they would be able to with the braided line or mono of similar strength.

In my humble opinion, the braided line's best purpose is for flipping and pitching in and around weed beds and submerged structure where some bullying of the fish is probably needed. Outside of that, the only other time I would use it is if I'm fishing for something toothy that might cut through other types of line.  For everything else, I use a Fluorocarbon.