A Fly Line is used to cast a fly. The weight of the line pulls the fly through the air. The heavier the line the larger the fly that can be comfortably cast.
Line weights generally range from 1 weight, for small flies, through 12 weight, for larger flies. As you can guess, 6 weight lines, falling in the middle of the range, are a general purpose line weight. Your rod must match your line weight. Most rods are designed for specific line weights and the rod's performance will suffer the further you stray from the designated line weight.
Most lines are tapered. The taper aids in casting and presentation. There are three general categories of line tapers, they are:
Double Taper (DT), Weight Forward (WF), and Shooting Tapers (ST)
Level lines are still available, but rarely used for anything other then as a running line behind a shooting taper fly line.
Double Tapers are aptly named. Both ends of the line have a tapered section, equal in length and taper too the other end. The line can be reversed if one end gets damaged and it is generally accepted that Double Taper lines Roll Cast better and Present Flies more delicately than other lines. Floating line applications are the prevailing uses for Double Tapered lines.
Weight Forward lines have a taper on the fly end of the line and a smaller diameter level line configuration on the reel end. The transitions point between the tapered section and the running section (level section) is located approximately 30 feet from the fly end. The level portion will then comprise the rest of the fly line's length. The Weight Forward taper is the easiest to learn with. Although a weight Forward line does not Roll Cast as well as a Double Taper line a Weight Forward line will normally cast further and cast easier into a head wind. Weight Forward lines are available in many styles (floating, sinking, sink-tip, & intermediate) and are the line of choice for the majority of anglers.
Shooting Tapers are a special case. These lines are short (just the head section) and attach to a separate monofilament running line or use a level fly line for a running line. Shooting tapers will allow casts of 100 feet or more and are commonly used by steelheaders and float-tubers. Shooting tapers give the Steelheader the distance they want and shooting tapers allow float-tubers to changes lines without having to re-string the rod.
Fly lines are further grouped by their buoyancy. (ability to float or sink at a predictable rate) There are three general categories of line buoyancy, they are:
Floating Lines (F), Sinking Lines (S), & Sink Tips (F/S)
Floating Lines are the most popular lines. It is the one used for 85% of all fishing, including the fishing of many sinking flies. (see Nymphing with a floating line) The higher a line floats on the water the easier it will be to pick up and cast that line. One tip for you is keep your line clean - dirt sinks line!
Sinking Lines are rated by the speed at which they sink. Intermediate sinking lines only sink 1-1.5 inches per second and are commonly used when shadows cast by high floating lines will spook fish or when surface wave action takes control of your fly line. Most other sinking lines fall into either Class II (1.75 inches per second), Class III (3.5 to 4.5 inches per second) or Class V (5.5 to 6.5 inches per second) and are Density Compensated to assure that the tip of the line sinks before the belly of the line.
Sink Tip Lines are a combination of both floating and sinking lines. The tip section of the line (usually 6 to 15 feet) will sink while the rest of the line wants to float. This arrangement will get you further down the water column, yet still give you some of the benefits of a floating line. (ease of mending, ease of pick-up)