In case you aren't clear on the difference yet, here is a screenshot showing how to navigate between the Visual or HTML editor for posts and pages:
If you intend to work primarily in the WordPress Visual Editor, there are a few points to keep in mind.
WP can create XHTML on the fly, making it nearly as easy to update a web page as it is to type an email. Making website administration available to people who don't know XHTML is a great leap forward for usability on the Web.
Unfortunately, such auto-generated code can have many undesired formatting issues. Ultimately, there is no substitute for a basic understanding of the structure and syntax of XHTML.
Even if you plan on primarily using the Visual Editor, there may be times when it is necessary to take a look at the raw code. One of the most common errors is the accidental inclusion of unnecessary styles. If the Visual Editor is acting strangely, checking the code for extraneous tags is the...