im sorry this is going to be a really noob question

ok ill start with my car model, 2007 impreza 2.0i
im in the market for 18" rims, my factory ones are 15", i heard going to 17" is alright without speedo adjustment but 18" u need to get it adjusted? is this right?

also.. what are my limitations on the rims, offsets etc..

ive read 5 x 100 and 5x 114.? thats 5 rims and what is the 100?

also ive read 100 x 8.5, is 8.5 the width of the rim?

and offsets.. + offset is so the rim sticks out from the tyre and - offset is kind of like a dish? what is my max (or min? offset so i dont need spacers ? (what else is there?)) or guard rolling or something

thanks heaps

edit: also ive seen some 240, 235 etc? 275?!