Comments allow you to engage in a conversation on the back of a card.
You can add comments to a card via the "Add Comment" textbox in the "Activity" section of the card detail.
Delete comments by clicking the "delete" link beneath the comment. You can delete any comment you make on any board. Board admins can delete public 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.