Microsoft Teams allows for 2 layers of hierarchy. At the highest level, you have a Team. Teams provide a great opportunity for a single place for collaboration for smaller groups of people.
The membership for a Team is always held at the Team level. Team members can then be added to channels within the team.
When to Create a Team
Teams are great for a smaller number of people that need to collaborate on a project or a portion of the business such as a depart or a a project that also has a lifecycle attached to it.
If you find that you have a common purpose where everyone needs to work on the same content and everyone should have access, that is when you should create a Team.
When to Create a Channel
Channels are great when you need to separate out content to a subset of the members of a team. If you think about a project, it may have different teams such as a development team. The content and messages that they have may not be relevant to the entire team, but they are apart of the project. This is the perfect opportunity to use a channel instead of creating a separate team as the content is related to the parent purpose.