Using HTML as the format of your messages offers you the ability to send formatted text 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 using the toolbar buttons shown below.
The first three buttons 
let you cut, copy, and paste text. Read this to find out more: Using Clipboard.
To change the format of the message to plain text, use the "Format" drop-down list.
If your message already contains text, it is automatically converted after you change the format.