Office 365 Groups
To the uninitiated (and sometimes even to the initiated), Office 365 Groups are perhaps one of the most confusing parts of Office 365.
You're probably best to think of a Group as simply users who have common access to a set of things in 365.
What, like a Security Group? Well, not exactly - because when you create a Security Group in 365, you give it a name, and you add some users, and that's about all that happens.
When you create an Office 365 Group, you give it a name, and you add some users, sure - but then, various things get created for that Group.
The confusion starts here.....
The things that get created will depend on how/where the Office 365 Group was created.
If you use Yammer, and you create a Group there, it will create:
A Yammer Feed for the Group
A SharePoint Online site (for file storage)
Or, you can create a Group from Outlook (which is the most likely way that a regular user would do it) - if you do that, these get created:
An Outlook Email Inbox (with its own email address)
An Outlook Shared Calendar
A SharePoint Online site
A OneNote Notebook
A Planner Plan
Or (because 365 is all about giving you choice), you can create a new:
Planner Plan - which actually first creates a Group, then a Plan, an Inbox, a Calendar, a SharePoint site, etc.
Team - which first creates a Group, then a Team, an Inbox, a Calendar, a SharePoint site, etc.
Ultimately, you can use some / any / all of these things to get work done - but at a basic level, that Group is some people with something in common - a department, like Finance; working on a specific Project; a social thing, like a company sports team.
Most people are going to have a preferred way of working - and in broad terms, Groups align like this:
Yammer for discussion-centric working (less real time than Teams)
Outlook for email-centric working
Teams for chat-centric working (more real time than Yammer)
Further information, along with a really good graphic showing how all these products hang together in the context of Groups can be found here: