Team Sites
A Group or Team Site a SharePoint site associated with an O365 Group and provides storage for the Group. Whenever a file is shared with the Group, it is added to the “Documents” library of the Team Site. There are many avenues that result in adding a file to the Team Site. For more details see Document Library.
Besides providing file storage for Group, the Team Site itself can be customized just like any other SharePoint site. Additional content such as Lists, Calendars, and additional Libraries can be created.
Permissions: Members of the O365 Group automatically have read/write access to the Team Site. However, others can be granted access to all or part of the Team Site without having to make them Group Members. They would not have access to the Group Mailbox or other resources owned by the group.
Use a team site when you want to collaborate with other members of your team or with others on a specific project. With a team site, typically all or most members can contribute content to the site and the information is limited to only the members of the team or project and specific stakeholders. Additionally, team sites are connected to groups and a Teams channel. This makes it easy for your team to have group permissions, as well as a Teams space to chat, meet, and share files.