What's the difference between a Project, ToDo or Timesheet?

ToDos are something you would set for yourself or another employee to complete, which doesn't directly have any progress affect on a project.

Projects/Project Tasks are a collection of phases/milestones. If you need to, you can set your billing model of a project to a fixed price, so that Timesheets which are lodged against that project are simply kept for reporting purposes and the client just gets billed an agreed amount.

Timesheets should be recorded to keep track of actual work performed.