Comments panel allows users to discuss Tasks. It can be accessed through the Task Details panel:

Comments button displays a total number of comments for a task.
To post a comment just type a text, then click Post Comment button or press Ctrl-Enter.
As a comment is posted, an email will be sent to all task watchers (see Task Details page).
Please note, that posted comments cannot be edited.
Any project member can delete comments made by him/her or by other people.