IRC stands for Internet Relay Chat. It's a protocol just for chatting with people, (one on one, or in rooms of people) kinda like http is a protocol for web browsers, or torrents are a protocol for sharing files.

To use the protocol, you just grab a program for it. mIRC is the most popular and has been for years. It's free. Once you run it, you pick a nickname, and a server, there are thousands, and connect, pick a chat room, and chat away. Be careful though, it's notoriously seedy. i.e. warez, porn trading, porn passwords, cracks, mp3 sharing, etc. It's generally used by bored high school and college kids, but there are still some regular, legit rooms (called "channels" on IRC).

It can also be slightly confusing at first, because it uses it's own commands, for example, to join a chat room (channel) named "chat", you would type "/join #chat", etc.

It's not really designed for video sharing or anything like that, just basic chatting, but you can transfer files (like pics) to each other.

GeekDrop uses IRC as it's chat room. You can try it out by clicking the link in the menu to the right that says "Live GeekDrop Chat".