You're going to see a lot of people on this site try and tell you about fraternities without having ever been a part of one. To listen to them would be like asking someone who has never been to Paris what Paris is like. You'll get an answer based off second hand experiences and representations in media, and you won't have any idea what it's really like to be in that city. Similarly, you'll have no idea what a fraternity really is.
In some cases you'll even get some pretty pathetic people, some of whom are already rearing their head in this thread, who spew some malicious and untrue shit. I tend to think in those scenarios that those people had a bad experience with/at one and are permanently jaded and judgmental against them.
Can some be full of scumbags? Sure they can. Like literally any other institution in the world, there will be scumbags. Do most party? Absolutely. But I know a fuckload of other people who weren't in a fraternity that came to our parties and partied their asses off with us. I know people who threw their own parties that I went to and partied my ass off at. I knew kids in our fraternity that hardly partied at all and instead studied their assess off. Partying isn't what being in a fraternity is about, partying is an activity. That the majority of college students do. I would have done it even if I wasn't in a fraternity.
The truth is that there is no catch all answer. It'd be like asking what an American hockey team is like and expecting a response that describes NHL teams, little league teams, high school teams, college hockey, town leagues, etc. all in one. Yeah they all fucking play hockey, but that's an obvious answer. If you're looking for the obvious answer, fraternities are like men's clubs. Some party, some don't. Most are secretive. Most have a process to join that could range from a cake walk to some watered down Army boot camp shit. Anything beyond that though and you'll soon find that every place is going to be extremely different. I was in a fraternity in bumblefuck nowhere in upstate NY. At our biggest we had 35 members. At our smallest we had 15. My experience was radically different from the guys with 100+ members at bigger schools. Every institution is going to be unique to the people and place it is located in.
You won't really know what one is like until you go there and meet some of the people who are a part of it. At their worst they can be like what you see in the media. At their best they can be like this post earlier in this thread. To me my fraternity brothers are my family, they're my best friends, they're some of the people I care most about in the world. I don't know what I'd be without them, they have shaped who I've become for the better. I think if I had to really try and answer what is a fraternity with one word or something, family would be as close as I could get. It's like becoming a family with a bunch of like minded guys during your stay at college, and for most people for the rest of your life.