So, the basics: manufacturers are always working to improve fly lines & the technology used to develop them. Sometimes the brand new "hot thing" in fly lines actually is an improvement. I know this for sure - lines are a zillion times better than they used to be even just twenty years ago. They float & cast better & hold up to use a lot more than they did back then. You also have probably 10 times the number of fly line companies than you did way back when, each offering many more models of lines for the angler's different needs. New types of lines are always being offered & I don't even try to keep up with many of the latest innovations. Still, I do keep my eyes open & you should too. Let's mention a few of the time-tested types of, or designs, of fly lines:
1.)Double Taper (DT): just as the name implies, these lines have a smaller tip end that grows in diameter until it's the appropriate size for it's designated weight. This smaller tip of the line is less bulky & lighter in weight allowing for the line to be cast gently onto the water with a minimum of disturbance. This tapered section will vary depending on the manufacturer, but it generally runs, say, the first 8 to 15 feet of the line. Many anglers use a double taper line & for good reason. They are easy to cast & give the angler a much needed advantage in fly presentation. Plus, the line is tapered on both ends, so when one tip is worn out you can just remove the line from the reel & turn it around. This gives the line (in theory) twice the life span.
2.) Weight Forward (WF): These lines are also popular because while they handle most fishing situations well, the heavier front section of the line allows for longer casts with less effort. Contrary to what some say, an experienced angler using a WF line can present a fly just as gently as if using a DT line. Basically if you're constantly needing to get your line out there a few more feet, or you're always fighting the wind where you fish, you might want to consider using a WF line. However don't make the mistake of thinking that a WF line is the answer to your casting problems. It can help you, but a good casting technique is the answer.
3.) Sinking Lines: These are particularly good for fishing deep lakes. Before these lines were developed, fishing in the really deep waters with a fly rod was difficult if not impossible. They do require a little adjustment in casting in that you can't just pick up the line & back cast it. The line is too deep for that, so you must strip some line in to get it up out of the depths before you start your cast. I consider these a specialty line for a specific fishing situation. I would NOT recommend them for normal fly fishing in streams, rivers, ponds, etc.
4.) Level Lines (L): Just as their name implies, these lines have no taper to them at all. They are the same diameter throughout their length. Most anglers will find these lines much less pleasant to cast than either DT or WF lines. They definitely won't get you the extra distance in your cast. So, why have them? Well, many years (decades) ago it was suggested that these lines be used by beginners. This is because they were much cheaper & would allow you to have a decent line at a reasonable price. Of course manufacturing processes have changed all that & now DT & WF lines can be made more economically, essentially killing that line of thinking. These level lines are becoming a very rare thing today. To me that's a shame because they did have some very good uses. I used to use them when warm-water fly fishing for bass, bluegill, etc. The level line was great because casting long distances & careful presentations wasn't an issue for these fish & if I stepped on the line in the boat or in the mud I didn't worry about hurting an expensive line. I hope levels lines do make a comeback.
Like I said, there are always new types of lines coming out. Things like "shooting heads" or lines designed for specific species of fish. For general fly fishing though, I like a nice DT floating line myself. It suits the type of fishing I do most often in my area. If you're looking for a new line, but aren't sure you have the experience to know what to get I suggest either a DT or a WF line from a reputable manufacturer. Whatever you do, just don't get the cheapest line available.