Comments allow you to engage in a conversation on the back of a card.
You can add comments to a card via the "Write a comment..." textbox in the "Activity" section of the card back.
Editing & Deleting
Edit or delete comments by clicking the "edit" or "delete" links beneath the comment. You can edit or delete any comment you make on any board. Board admins can delete any comments made on their boards. Deleting is permanent.
You can mention people in comments by using "@" followed by the user's name. The mentioned user will get a notification.