The Audit log will show you the operations you performed on the entity like projects, tasks. It will show you details of the respective attributes modified by whom and date/time.

To see the project's audit trail :

Navigate to a project and from the More option on right click on Audit Log.
You will see the project's attribute modified By, which attribute with old and new values, and date and time of the modification.

To see the task's audit trail :

Navigate to a project.
Go to the Tasks (List) report.
On the action's drop-down for the task, click on Audit Log
You will see the information like Modified By, attribute modified with old and new values, and date and time of the modification.