Activities record work completed in relation to a project.
Actions which involve time or cost will update those aspects of the project, such as remaining time, and these percolate up the project tree hierarchy for analysis.
Recorded actions also show in the user calendar.
If the start or end dates are omitted when recording an activity, then these will be calculated from the stated duration, and assuming the activity has just ended.
Events are also automatically recorded for changes to the project, such as status transitions or updates to key properties.