The Advanced Editor is the preferred tool for content creation and editing. Activate the Advance Editor by clicking on the double-arrow icon.
For most editing, leave the text editor in Rich Text Editor Mode.
If you need to view the HTML block for the page, select the Source icon in the upper left corner.
More detailed information about the Advanced Editor can be found here: http://docs.cksource.com/CKEditor_3.x/Users_Guide