A project defines to which charity and for which area the fundraised money will used. All charities have a project for the "General Work" of the charity, and some also have other projects that are dedicated to more specific campaigns. All fundraisers must choose one project to fundraise for.
An event is a gathering of fundraisers/projects. It can be connected to one charity, or multiple. The event is an umbrella, and shows the total amount fundraised of all the projects and fundraisers under it. By creating an event, you can also open up for the creation of teams. You can read more about how it is all interconnected here.