CKEditor allows user writing in the editing area and also provides the ability to view how it looks in HTML code.
Sample text written in the editing area:
File:CKEditor example source.png
To see the HTML code user should press button on the toolbar.
Image:CKEditor_example_source2.png
To go back to the normal text press again.
Editing the code in HTML language is a very difficult feature and it should be done only by advanced users. If you do not have experience in HTML type the text normally.