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.
Add an attachment
Add an attachment to the activity feed while making comments on a card by clicking the "Add an attachment" link and selecting the file that you would like to attach to your card.
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(s) will get a notification about the comment.
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.
Add a card link
To quickly add a card link to a comment either click "Add card..." or type " # " and then begin typing the name of the card that you would like to link to in your comment. A window will populate with the card titles that match your text and you can select the card to link to.
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 a comment is permanent.