Using HTML as your message format offers you the ability to send messages with animated graphics, pictures, multimedia objects, and anything else that a Web page can contain.
While editing in “HTML” mode you can use the formatting commands by using these toolbar buttons:
The first three buttons 
are used to access the clipboard. Read this to find out more: Using Clipboard.
To change the format of the message to plain text use the dropdown list located on the upper toolbar:
If your message already contains any text it will be automatically converted to the new format.