If you're new to Teams, watch this short video, Welcome to Teams. Teams is built on Office 365 groups, Office Graph, and the same enterprise-level security, compliance, and manageability as the rest of Office 365. Teams leverages identities stored in Azure Active Directory (Azure AD). To see where Teams fits in the context of Microsoft 365, check out this architecture poster: Teams as part of Microsoft 365
When you create a team, here's what gets created:
Don't miss org-wide teams, a special type of team that pulls in every user in your organization and keeps membership up to date with Active Directory as users join and leave the organization.