Difference between a Group and a Page?

Hello

Sorry for this dummy question but I see that I can create a Event, a Discussion in a Page.
This is more or less the key feature of a Group.

What are the criteria to choose between create a Group or create a Page?

Beyond that, is a 'read me first' documentation already exist which explains the main 'object' we propose: Group, Event, Blog, Page, Article, etc.
It will save us a LOT of time to avoid to re-invent the wheel.

Thanks
Henri