All the completed tasks/actions/activities related to the selected opportunity are displayed and can be tracked within the Profile tab.
Here you can not only view the information about the completed tasks, uploaded documents, sent/received correspondence etc. but also add some events related to this opportunity: note, email, phone call, appointment. Just select an event type from the list, set its date, add a description and click the Add This Event button.
To notify other users about the created event click the Add user link and select the desired users from the list before adding this event.
All the selected users will receive a notification about a new event added.