Line weight
Fly fishing line weight is ranked from 1 to 15, with 1 being the lightest and 15 the heaviest
Lighter lines are suited for delicate presentations and for casting light flies
Heavier lines are best for casting large, wind-resistant and heavy flies
Line weight is the easiest to select since this should be matched to your rod and reel
Fly fishing requires a balanced system so match the reel and rod. If you don't, you will hurt your casting accuracy and efficiency.
A 5 weight reel matches up with a 5 weight rod, so it follows that you should select a 5 weight line
Some manufacturers give a leeway by saying you can go one above or below this, say a 4 weight or 6 weight line with a 5 weight reel and rod. If you want to be safe, though, match the line, rod and reel exactly.
Your fly fishing line weight should also be selected based on the fish you want to catch
You'll need line weights from 1 to 7 lbs for lighter fish such as panfish and most trout

Line taper
To help you cast more efficiently most fly lines are tapered. This taper varies in weight, diameter and thickness.
The taper is listed as an abbreviation by the manufacturer.
Weight-forward (WF) taper
These are the most popular and the best choice if you are a beginner
The first 30 feet or so of line is heavier because of its tapered front end
The rest of the line is thinner and is known as the running line
The weight-forward line helps with long casts and better precision even in windy conditions
Bass bug/saltwater (BBT) taper
This design helps with heavier flies, hence its use for catching feisty bass or bigger saltwater fish
Double taper (DT)
DT fly lines are preferred by seasoned fly anglers
These lines work especially well in making delicate presentations on small- to medium-size rives since the belly is at the center, with both ends gradually tapering
This makes the line highly economical too because when one end wears out, you can turn the line around and use the other end
This line won't cast as far or provide as much wind resistance as a weight-forward line
Shooting taper (ST)
ST lines cast farther than other lines so they are designed for fast-running rivers and in extreme wind conditions. The line portion (front section) is stout and short to form a casting loop.
Level (L) taper
These lines are uniform in diameter throughout, making them the most economical
If you are a beginner don't try to save money this way. Level taper lines are the most difficult to cast so they really are best used by seasoned veterans, primarily for fly fishing with live bait.

Density
How your line behaves on the water depends on its density or line type, which affects its buoyancy
With different types of line available, consider buying an extra spool when you purchase your reel. That way you can spool various types of line and switch lines to meet conditions.
There are four choices, and each carries an abbreviation, included below, to identify its density:
Floating (F) lines
Floating lines are good for beginners since they are easier to cast and handle
Floating lines also are a must for dry flies, but they can also work with wet flies, nymphs and streamers that are fished several feet below the surface
Intermediate (I) lines
These are a little denser than water so they sink slowly to present a fly just below the water's surface
These lines work well in shallow, weedy lakes and in choppy waters where you want your line to stay below the choppiness
Floating/Sinking (F/S) lines
These combine the two characteristics the five foot to twenty foot tip or front portion sinks to present the bait while the balance of line floats on the water
Manufacturers display the depth and speed that the front part of the line sinks