Once on B.net you enter a chat-like environment. There are public channel where you can chat, and there are private channels that you can go to if you want to meet up with your friends. I usually create because I've got a fast connection. Either way you do it you will end up with this screen. StarCraft is a real time strategy game, or RTS, so there are different maps and types of games you can play, each game creator has to pick the map and type of game he wants before it can start.
Creator can also ban players or add computer players.
Once eveyone has joined then the games can begin. I also have a page of screen shots from StarCraft that you can look at to see what the in-game looks like. However this is just one game and one server. There are many others all over the internet. Another popular one that I don't use is called kali.net.