GringoTim-
NL means that there is no limit to the amount you can bet. 1/2 is the minimum amount you can bet. Before the flop and on the flop, you can bet anything from $1 to all-in (all the money you buy-in for at that table). On the turn and river, you can bet anything from $2 to all-in. That's if you are the first to bet. If somebody bets before it's your turn, you can raise them any amount at least double their bet. You can always go "all-in".
It can be a very fast game. You can lose your shirt in seconds.
You can also win very fast. 
I play online regularly, but at a different site. Nothing against ACR, but I tried it and it wasn't for me. Just as an example, earlier today, I bought in for $100 and played 1/2 NL for literally 20 minutes. I cashed out for $403. I won 2 big hands (ace nut straight and ace nut flush).
Very very fast game.
On the other hand, fixed limit games allow you to only bet (or raise) $1 before and on the flop. You can only bet (or raise) $2 on the turn and river. The pots in 1/2 limit are generally pretty small (under $10) but I have won a couple large pots (over $40). Most players in limit games chase a lot with nothing because it doesn't cost them much to do so. In a NL game, if you think somebody is chasing, you can put him "all-in"
For beginners, I'd recommend low limit (0.25/0.50 or 0.50/1.00) games once you get used to the controls and everything.