to define custom light workflows on opportunities.
Activities are linked to mail.message.subtype through inheritance.
This means that the follow mechanism works for activities. When
an activity is done, a message with the matching subtype is posted
on the opportunity.
Activities are internal by default. They are only visible by
employees.
A shortcut in the 'log a note' chatter option allows to post a
note linked to an activity (subtype).
Martin Trigaux