Oh good. Question. What is the difference between a leader and tippet?
The leader is tapered mono; either a single piece or gradually smaller/lighter diameter sections tied together (it depends on what you're fishing for, and what type of flies you're using to some extent). I prefer a single-piece tapered leader for salmon, trout, and panfish. The taper helps lay the leader and fly out nicely in gentle S-curves (important for dries, and it helps wets sink too). The end of the leader is actually parallel, and can be used without a tippet if desired.
The problem with not using a tippet is that as you clip the end of the leader off to change flies etc, you eventually run out of the parallel material and end up on the taper, which gets progressively thicker and doesn't lay out as nicely on a cast (and since it's getting heavier, it's getting easier for finicky fish to detect).
The tippet is a small-diameter parallel mono (not tapered). You tie a few feet of it on the end of your leader so that as you clip off pieces to change flies you aren't shortening/wasting your leader. When the tippet gets short you replace it, but have only made one cut against the leader for 20-30 fly changes. Your leader lasts MUCH longer that way.
Replacing the tippet is much easier and cheaper than replacing the leader. Using smaller diameter tippet allows you to play with the overall taper as well. The overall length of leader/tippet matters too. Too long and it'll cast bad and tangle/wind knot easily. Too short and you'll probably spook fish.
The knot you use to tie your tippet to the leader is absolutely critical. It MUST be tied well, or it'll be your weak link and snap when a decent fish hits. I like blood knots, moistened with saliva, but there are other options too.
