While writing an article in Joomla 2.5, one can either use WYSIWYG editor or you can edit the HTML of the article directly as the HTML editor allows you to view all of the HTML code that makes up an article rather it will hide all of the HTML code in case of your WYSIWYG editor and in essence it allows you to publish your article after previewing it. Here is how to switch between WYSIWYG and HTML Editor in Joomla 2.5.
Normally the HTML editors are used by the advanced users while working with the articles as those same advanced users used the WYSIWYG editor as often as the WYSIWYG editor makes very easy to insert special characters such as copyright the characters into your articles.
Now if you are in the WYSIWYG mode then you can able to view the HTML page that has been appear, and vice versa.