From Joomla! Documentation
TinyMCE is the default editor for both front-end and back-end users. TinyMCE is a WYSIWYG (what you see is what you get) editor that allows users a familiar word-processing interface to use when editing Articles and other content. TinyMCE can be configured with three different sets of toolbar buttons: Set 0, Set 1, and Set 2. This is set as an option in the Plugin Manager for the 'Editor - TinyMCE' plugin. Set 0 is the most extended toolbar and is by default assigned to the Administrator, Editor and Super Users. Set 1 is assigned to the Manager and Registered. Set 2 is assigned to Public.
Learn about Toolbars and Accessibility of TinyMCE.
If 'No editor' is selected for a User, then a simple text editor displays. This allows you to enter in raw, unformatted HTML. You can use the toolbar 'Preview' button to preview how the HTML will display.
Note that the 'No Editor' option can be useful if you are entering in 'boilerplate' or custom HTML, for example to create a PayPal link. TinyMCE automatically re-formats and strips some HTML when a file is saved. This can cause complex HTML to not work correctly.
If this happens, you can temporarily change the editor to 'No Editor' and create the desired content. Note that if you wish to edit this content in the future, you should be careful to change your editor to 'No Editor'. Otherwise, if you open and save the content with TinyMCE, you may lose your custom HTML.