Users are the platform representation of individuals. These representations have a user id (unique identifier), an email address, a display name, and two flag: Enabled/Disabled and Space Admin/Non-Space Admin. No one can login to the platform without a user. Now, it is possible to set up Single Sign On (SSO) so not all a user's information (roles, organization, department, etc) be stored in the platform's space, but even with SSO, the person logging in must have a user in the space.
Users can have attributes and profile attributes, which can be defined per space. Users can also be members of teams.
Teams are groupings of users. These can be organizational teams the user is part of or teams that represent roles the user has. Both types of teams are useful because teams are part of how permissions can be defined throughout the platform.
It is possible to add a user to a team as a member and remove a member from a team. Both of these actions have webhooks that go with them, along with all of the webhooks you would expect:
Note that there is a user guide for the standard solution displays / management of users and teams here.