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 comments made by any non admins on their boards. Deleting is permanent.
Mentions and Emoji
You can mention people in comments by using "@" followed by the user's name or by clicking the "Mention a member" link below the comment input field then selecting the members you would like to mention. The mentioned user will get a notification. You can also add emoji to your comment from the "Add emoji" link beneath the comment input field or by using colons around your emoji term (ie. :emoji:). Business Class and Trello Gold subscribers can upload custom emoji.