Rich editing, also called WYSIWYG for What You See Is What You Get, means your text is formatted as you type. The rich editor creates HTML code behind the scenes while you concentrate on writing. Font styles, links and images all appear approximately as they will on the internet.
WordPress includes a rich HTML editor that works well in all major web browsers used today. However editing HTML is not the same as typing text. Each web page has two major components: the structure, which is the actual HTML code and is produced by the editor as you type, and the display, that is applied to it by the currently selected WordPress theme and is defined in style.css. WordPress is producing valid XHTML 1.0 which means that inserting multiple line breaks (BR tags) after a paragraph would not produce white space on the web page. The BR tags will be removed as invalid by the internal HTML correcting functions.
While using the editor, most basic keyboard shortcuts work like in any other text editor. For example: Shift+Enter inserts line break, Ctrl+C = copy, Ctrl+X = cut, Ctrl+Z = undo, Ctrl+Y = redo, Ctrl+A = select all, etc. (on Mac use the Command key instead of Ctrl). See the Hotkeys tab for all available keyboard shortcuts.
If you do not like the way the rich editor works, you may turn it off from Your Profile submenu, under Users in the admin menu.