TinyMCE is a WYSWYG editor that allows drupal users an easier way to create formatted input to drupal.
By selecting the "Filtered HTML" input format, TinyMCE is activated. Different formatting options are implemented as buttons arrayed across the top of the input window. Headings, links, ordered and unordered lists and a variety of other styles can be implemented without having to remember (or type) cumbersome html markups.
If you later need to edit the html, this can be done by entering edit mode and selecting the "Full HTML" input format.