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.
I got it for my son for $13 when it was still in beta and a couple of years on he still loves it. I can't count the number of games he has played for a week then forgotten about. So for a game that isn't crazy graphics orientated but can still hold the interest of an 8 year old (now 10), I'd say it was worth it at twice the price.