Each IRC network is composed of hundreds, perhaps thousands, of channels where individuals with similar interests can chat realtime with one another. Channels are dynamic by nature. A channel is created the first time someone enters and destroyed when the last person leaves. On some networks, such as DALnet and Undernet, channels can be registered after creation. Registration allows the creator to become a channel operator every time he logs on to his channel.
Registration assigns ownership of a channel.
Many IRC networks, including the ever popular EFNet, don't have channel registration. When you leave a channel, you leave all your privileges in that channel as well. Hackers love dynamic networks like these because it allows them to take over channels. Hackers will force all legitimate users out of a channel until they are all that is left. When they are the only ones in the channel, they can op themselves.