So here are the basics of fly lines:
Length - Typically, a fly line is 80 -100 feet in length. It varies widely and is dependent on the manufacturer and the type and purpose of the line
Taper - There are three basic tapers that are most common:
Weight Forward (WF),
Double Taper (DT)
Level (no taper and not very common).
There are a few hybrids like a "triangle taper"
Weight (wt) - Fly line weight is traditionally based on the weight of the first 30 feet of the "working portion" of the line and is measured in "grains". (A grain being the unit of measure.) The "working portion" measurement omits any tip on a taper in the fly line. Line weight are typically available from 2 wt. to 12 wt. and some are available up to 15 wt.
Fly line types - There are three primary types of fly lines: floating lines, sinking lines and sink tip lines (a floating line that has a sinking portion on the tip of the line. The length of the sinking portion varies in length depending on your fishing needs. A longer sink tip allows you to fish deeper than a shorter sink tip. There are also "shooting heads"...more info below.
Floating lines - Floating fly lines do just that…they float. Floating lines are the most versatile type of fly line. They allow you to fish dry flies, nymphs, streamers, rivers, lakes and everything in between. Floating lines come in either a weight forward or double taper. I would recommend a floating line when learning how to fly fish. If you need to get your fly deeper than 4-5 feet, your should probably consider a line that will sink.
Sink tip lines - These lines are used primarily for fishing streamers. They are basically a floating line with a tip section that sinks. The tip section can vary in length and weight depending on your particular need.
Sinking lines - As you might have imagined by now, it gets even more complicated. Sinking lines have numerous ratings or sink rates. They are rated by how fast they sink and this is measured in inches per second (ips). So here are the common ratings for sinking lines.
Intermediate = 1.5-2.0 ips (2-4′)
Type I = 1.5-2.5 ips (2-4′)
Type II = 1.75-2.75 ips (3-6′)
Type III = 2.5-3.5 ips (5-10′)
Type IV = 4.0-5.0 ips (10-20′)
Type V = 4.5-6.0 ips (10-20′)
Type VI =6.0-7.0 ips (15-25′)
Type 7 = 7.0-8.0 ips (20-30′)
If you want more detailed information on sinking lines, here is a link to a good article I found on the Demystifly web site...click here...
Shooting heads - A shooting head's purpose is to allow you to make longer than normal casts where you need to "cover" a lot of water and do not need a delicate presentation. Examples are steelhead/salmon fishing and many saltwater situations where you are not "sight" fishing. A shooting head is simply a short, heavy section of fly line (usually sinking and approximately 30' in length) attached to a very thin running or shooting line that is a "slick" monofilament or similar material. You can think of shooting heads as a weight forward line on steroids. Shooting heads are typically 2 line weights heavier than the fly rod being used. Some fly line manufacturers are now making what are called "integrated head" fly lines that incorporate both shooting head and running line.
Advantages of a shooting head - quick loading, requiring minimal false casting, they can "punch" through the wind, they are heavier and shorter so can handle larger flies than normal, very good for covering a lot of water (cast further) and when setup properly, you can quickly change to a heavier or lighter head as situations change. There are some disadvantages as well: mending line is impossible, there is often a "hinge" point where the head meets the running line and there is very little "delicacy" in presentation of the fly, You also have a lot of running line that you need to strip in before you make another cast.
Casting a shooting head is certainly different but not especially difficult. You must get the entire shooting head (usually 30' or so) just outside the rod tip.Use a roll cast or two to get the shooting head in the air and cast. Those that use shooting heads often use a double haul cast to develop line speed more rapidly
Fly line care and maintenance - Over time, a fly line will pick up dirt and grit and should be cleaned periodically. A good way to clean your fly line is with warm water and a small amount of dish detergent. Let it soak for a few minutes and then use a clean soft cloth and draw the fly line through the cloth applying some pressure. Then, use another clean cloth to dry the line. Fly lines also need to be periodically "dressed". There are several line cleaner products on the market and they work well. Personally, I use Rain-X that I buy at the auto parts store. One small container will last years. Line dressing is like waxing your car...it makes it "slick" again and can certainly help with your casting as the line more easily slides through the guides.
Fly line backing - Happily, the subject of fly line backing is pretty simple. Backing is almost always a braided nylon or dacron material that does not stretch. For the typical trout/bass applications it is in the 20 lb. range. When it comes to larger species, it gets much heavier...up to 50 lb. or even more. Backing is typically tied to the reel with either a uni knot or arbor knot.