A channel is a conversation organised around a subject rather than around a set of people — a team, a project, a topic that outlives whoever is currently in it.

That is the difference from a group chat. A group is the people in it; a channel is the subject, and people come and go around it. Use a channel when the conversation should still make sense to somebody who joins in three months.

Two things worth knowing. Channels exist only in an organisation context — there are none in personal chat. And a channel can be made public, which lets colleagues find it and join themselves rather than waiting to be added.