Under any question or answer you can post a comment. Simply click the 'Comment' button and a new field will appear to place the comment.
In addition you can add a comment to any post, whether it's post about a video or just a post on a wall.
Once the 'Comment' button is clicked, a notification and email (if opted) is sent to the poster regarding your comment
Editing a comment can be done by simply hovering over the comment and clicking the down arrow, then selecting 'Edit'.
Deleting a comment can be done by again hovering over the comment and clicking the down arrow, then selecting 'Delete'.
A pop up confirmation window will appear and asking if you want to delete the comment.