An activity is the type of work your perform on your projects. For example, if you have a project named "Washing machine" that consists of building a new revolutionary washing machine, activities that you could define could be "Research", "Design", "Tests", etc.

A default billing rate can be entered for each activity type. For example, you could bill your customer $100 per hour when you do design but only $75 per hour when you do tests.

The billable rate per activity can be modified for a specific project by indicating it at the project level or according to your custom criteria using the billing rules.

You can even define user defined fields if you need to keep more information pertaining to your activities.