Office 365 Groups is a service that enables teams to come together and get work done by establishing a single team identity (managed in Azure Active Directory) and a single set of permissions across Office 365 apps including Outlook, SharePoint, OneNote, Skype for Business, Planner, Power BI, and Dynamics CRM.
Office 365 Groups is a service developed with collaboration in mind. It works with the Office 365 apps you use already so you can collaborate with your teammates when writing documents, creating spreadsheets, working on project plans, scheduling meetings, or sending email.
Additionally if you are using your Office 365 group in Outlook or Outlook on the Web, you will get an Inbox for that group that contains all of the group messages and a shared Calendar for scheduling group meetings and events. If you're using your Office 365 group in Yammer, you will use Yammer conversations to exchange those messages among your group.
Check out this video to see Office 365 Groups in action.