I'm trying to get this straight in my head so here is what I've deduced so far - please correct me where I'm wrong
You retrieve differently according to what type of fly you're fishing - trying to impart the most natural/take inducing movement you can.
Lures - are "stripped" which I think means a very fast retrieve so they act like the small fish they are imitating, darting about etc.
Traditional winged wets - can be thought of as lures, a fairly fast retrieve most dont necessarily imitate anything specific but are attractors and the movement helps draw the fishes attention.
Nymphs/buzzers - a much slower "figure eight" retrieve which is as much about getting them to rise and sink in the water to act like the stage of insect they represent as it is bringing them in and also it keeps the slack out of the line.
Dries - again the retrieve is slow and possibly a bit jerky, trying to generate movement in the surface film like an insect trying to get free.
I suspect there are types of dry retrieved fairly quickly as well - basing this on a bug I saw that really scooted across the surface film to get to the bank, generating a large wake. Wasn't close enough to see what it was though.
I'm coming from a mainly stillwater perpective so haven't included the effect of the current but the objective there as well as the above is to get the fly to act naturally in the current without the line causing it to act oddly.
There must be other things I'm missing here so please fill me in.