A team is a group of people within your organization working together for something specific. You may have a team for a specific project, for your entire department, or maybe even your entire organization. You can be a member of any number of teams. For example, your organization may have separate teams for each department, with some people being members of multiple teams.
Teams are made up of channels, which are conversations on specific aspects of what the team is working towards. For example, a Marketing team may have one channel for work on the company website, and another channel to coordinate for an upcoming trade show. A team can have any number of channels, including the default General channel. Everything that happens in a team—conversations, file sharing, or meetings—takes place in a channel.