Projects - In GTD, project management is fairly simple. Projects are just a way to group actions that work toware a common goal.
Contexts - Contexts are a way of associating similar actions by the method or location that they are accomplished. Some examples: Phone calls > @Phone; Email, web, or computer tasks > @Computer; Home Improvements > @Home; (the @ symbol is not required)
Contacts - Contacts allow you to track the individuals or businesses that you have delegated actions to. You can even share and delegate items to another account if the contact is a Vitalist user.