The German standard DIN 69 900 describes activities as "components of a process " with the following characteristics:

• They have a duration.
• They have a defined start and a defined finish.
• They are processed without interruption.
• Resources are needed to execute them.
• They incur costs.

The Project System distinguishes between the following types of activity:

• Internally processed activities
• Externally processed activities
• General costs activities

You can break down the activities into activity elements for more detail.