Follwing the channel name you will see the number of people on it as well as its topic. A channel is a 'place' on IRC where group conversations occur. People can join the same channel and see each other. Depending on its topic and time of the day a channel can be VERY crowded. Channels can alsobe quite chaotic, or calm.
Channels can be open to everyone but also closed and private and only open to friends.
Channels on IRC are dynamic in the sense that anyone can create a new channel, and a channel disappears when the last person on it leaves. Now that I've decided on a nice channel. Once you get to the channel, you will see people talking. Unless you're familiar with the channel you may want to sit and watch it for a minute or two to see what the conversation is about. In the channel window that opens once you join a channel you'll see an alphabetical list of people that are on the channel on the right side of the window.