The "chopp" or "chope", as some of us brasilians (yes, that's an actual 's', not 'z') like to call it, is one of the cornerstones of the brasilian modern society as a whole. The Chopp is so fundamental that it became a very strong institution to the brasilian folk. That said, let's get to the facts:
Ethimologically, the word "Chopp" derives from, well duh, the German word Chopp (or Schoppen), that actually means 300 millilitres (something about 10 fl. oz, if my math is correct). Eventually the word Chopp became a synonym for a glass of beer. Something like pints and half-pints, but not quite it. You can order a pint of, say, Guinness or New Castle Brown Ale, but when you order a chopp, that's what you get: a chopp. Confusing? Read on...
The Chopp as a beverage (or booze, if you will):
As you already know, there's a thing called pasteurization. You know what it is for, so I won't get into the core of Pasteur's contribution to mankind. Basically, chopp is a beer that hasn't been pasteurized, therefore the chopp can get nasty quite rapidly (something about 10 days or so) and the beer (the pasteurized one) can go on unspoiled for a longer period of time. That's chopp in a nutshell: A non-pasteurized beer. So, if you ever come to Brasil and ask for a chopp, that's exactly what you'll get.
The social phenomenon called Chopp:
Brasil is a mostly a very hot place, and by that I mean *tropical* hot. Chopp is a very refreshing drink and many, many people like it. Besides, chopp is a social thing: people often gather to drink some of those. They don't say: "Let's go out for a couple of drinks". Instead, they say: "Hey, let's have a chopp!". It's a very light drink, unlike the famous caipirinha, and if you're used to it, you can have a dozen chopps without getting utterly hammered or, even worse, getting a mega hang-over the next day. And, of course, in a chopp-drinking environment things get a lot more friendlier, and that's usually a good thing. Having a chopp with your friends, co-workers, school buddies, etc. is a social event. People actually interact more, thanks to that heavenly drink.
When in Brasil, you might see a bunch of people hanging around (litteraly) the bars, standing on the sidewalks, wherever, just chatting and having a chopp. You might want to try it out, so go on, order a chopp and make conversation. You'll be very welcome.