System administrators may impersonate a user to view and perform functions of another user without the user's authentication information.
The impersonate icon is in the Administration > User List in the Admin column:
You can tell you have impersonated a user in the top right Nav bar:
And you Stop Impersonating from that same menu:
When you update an item while impersonating another user, in the History tab shows the impersonated user updated that task but the impersonator shows in parenthesis: