A summary activity is a group related activities in a schedule aggregated and displayed as a single activity. It is used to summarize the schedule information for a group of activities and helps the upper management to have a higher view of the project without getting into details.

Many Project Managers use the terms Hammock activity and Summary activity interchangeably; however, the objective of using them is different where the Summary Activity is simply a roll-up of activities defined in a logical way using the schedule relationships; and the Hammock Activity is grouping smaller unrelated sub-tasks that hang between two dates.