An IRC (Internet Relay Chat) Network is composed of one or more IRC servers (machines that run software IRCd) which are all connected to each other. These networks host chat "channels" that is, the rooms where conversations take place. When connecting to any of the servers on a specific network, you will see the same channels and people.
IRC networks are created, owned and managed by people and/or independent organizations. Worldwide there are hundreds, perhaps thousands, of these networks.