Like in any organisation some people have special tasks to do. To do that they come together in teams. Both groups and stores can have teams.
There might be a team for forum moderators, one for welcoming new members, another for kicking out naughty people who ignore the rules, another one to take care of the cooperations with stores. It's up to the admins of the community/sub-group to decide which teams they want. They can create a team and define what it lets you do when you are a member - in other words, the admins can give permissions to the teams (respectively to the team members). Some default teams are created for common needs.
A team can also have a special meaning among a relationship of groups. Think about the welcoming team for Berlin-Spandau which may want to discuss with the welcoming team Berlin-Zehlendorf. Therefore, they can use the common welcoming team Berlin that they are also part of.
Teams will have a wall attached and may have other funcionalities like the report module, depending on their permissions.