Sup! Newb questions incoming

[Kanade Kun]

Hello all~
I've been playing games for ages, though these games where its more timing based, i've only played OSU! and done decently well i'd think, not much to say about it.

Thought i'd drop some quick questions (well, hopefully they're quick) about the game itself.
After reading quite a bit of the forums last night, i realised i had no idea what most people were talking about, even after reading the faq and other things like that. The main things being: d5/d6 etc, as well as some words you use when describing your scores you get.
Oh, and this speed thingy, where you set it from 1~ to 2.5~, i really dont see a difference at the moment, and the only thing i got off reading the forums was it changes the spacing between notes?

alright.... that may be badly worded... But any help would be appreciated!