A project contains a group of datafiles. It is the central location for all your data and analysis.
A project page is the home base for all your data. All data is uploaded to a project. You view and work with your data from within a project. You invite people to a project to colaborate with.
You can create as many projects as you need and each one can be shared with different people who can have different access levels.
You can think of projects as folders. Everything that belongs within on project goes into that same folder. You can share the contents of everything in that folder, give rights to the people you share it with, or keep it private.