Office 365 Groups is a service developed with collaboration in mind. It works with the Office 365 apps you already use so you can collaborate with your teammates when writing documents, creating spreadsheets, working on project plans, scheduling meetings, or sending email.
Groups are similar to distribution lists, but they offer so much more. A distribution list is a set of email addresses. You can use the distribution list to send an email message or meeting invitation to all the addresses at once. An Office 365 group includes a distribution list but also includes a shared:
Document Library for storing and working on group files and folders
OneNote notebook for taking project and meeting notes
Planning tool for organizing and assigning tasks and getting updates on project progress
When you join a group, your email address is automatically added to the distribution list and you gain access to all group information. You don't have to wait to see group email or view a shared document. You join, you have access to everything, from everywhere: in the cloud, on the desktop, or on a mobile device.
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.