There are a number of types of fly lines, one of the common differentiators of these is fly line density. Fly lines have different densities, each density is used for a different method of fly fishing. At the highest level, fly lines either float or sink. When we break it down a bit further we discover that there are more than two types of fly line densities. Here we're going to cover four common types: floating, sink-tip, sinking and intermediate.
Floating Fly Lines
Floating fly lines are easy to understand, because they simply do what their name suggests, they float on top of the water (or sit in the water film). Floating fly lines are hugely popular for fishing dry flies or nymphs; they can also be used to fish streamers or wet flies. Floating lines are made with small air bubbles in their coating, making the line less dense than the water; this allows the line to float on top. You connect a leader to the floating line, to enable your flies to get down to the fish that is waiting. If you're learning how to fly fish then this would be the best place to start.
The sink-tip line is a variation of the floating line; again, as the name suggests, it is a floating line with a sinking tip. A sinking tip means that the density of the tip, or end of the line, is heavier than water and therefore the tip sinks into the water. So part of the line is floating on the water, but the end (the tip closest to your flies) is sunk in the water. This type of line helps to get flies down in the water quickly, but does not require the entire line to sink in the water. This can often be a great line for smaller, shallower rivers when you want to fish with your line in the river, but not too deeply - to prevent it will snagging on the bottom.
Sinking lines, when cast onto water, are designed to sink into the water.
Intermediate lines are sinking lines, but they don't sink as fast as true 'sinking line'. These lines vary in their sink rates, so it is important to understand the differences in brands of intermediate lines and their sink rates. Intermediate lines are ideal for fishing lake edges and shallower rivers as the line will have time to get down to the right depth, however it won't snag on the bottom like full sinking lines do. The retrieve (pulling in the line by hand) speed will determine at what depth the intermediate line sits.
The above lines densities are the four key types you will encounter, as you would have guessed there are variations of each type. If you are just starting to learn to fly fish then you'll need to understand what fly line density is going to suit the water you'll be fishing most of the time.