WYSIWYG = What You See Is What You Get.
With a WYSIWYG. you can create HTML using the WYSIWYG's toolbar. You don't need to go into the code.

Of course, if you want to mess with the code, you can. Just click "Source."
Drupal and Guides use the same WYSIWYG ("CKEditor," in case you were wondering).
"CSS files" are often called "Stylesheets"
Here's a definition of CSS from the W3C Schools:
Usually, CSS is stored separately from HTML files. That way, a single CSS file can be called by multiple HTML files.