From what i have read is that a Premium account is one you pay for and can play anything and everything. Change skins etc. And a non-premium is just for playing classic. Were it is very limited and i don't think you can go on other peoples servers and such. It's just a basic version of the full game to give you a feel for it.
Pay for it. It's well worth it
Oh and of course when you have paid for it you can play such epic things such as Technic/Tekkit. FTB (Feed The Beast) and much much more.