not a bad question. basically dry flies are bugs that hatch (mayflies,midges,etc). they float. then theres nymphs, they r the same bugs but before they grow wings and hatch they live underwater. basically you use these with a indicator or type of "bobber" as some folks call it. a wet fly is also the same bugs. just swung in the currents and pulled in with lil jerks or strips as we call it.
terrestrials are beetles, bugs, crickets ,ants , dragonflys, etc... basically anything that don't live in the water but falls in! streamers imitate baitfish,minnows, crayfish, leeches, etc. u strip um through the water like escaping prey. any other questions? feel free I was new once too! another thing is to read this site it helps along with this forum once you get the basic slang down. Fly Fishing "For Beginners - 101" FAOL
read this site and you will learn all the basics of fly fishin. by the time you read this whole page you will basically have it down! you can use all these types of patterns for many kinds of fish. basically look around where you live. what bugs and fish and crustaceans are there? if ya know that. well now you kno what to imitate.