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 ...