The CKEditor is used on the big text fields. For example, the Body field on basic pages.
Note that across the top there are a lot of style buttons. These should be similar to buttons found on Word documents. If you hover over any of the buttons, hint text will show up explaining what the button does.
The Styles drop-down is there to give you some pre-configured formats.
The Format drop-down allows you to make something a header or title vs. normal text.
The link button allows you to turn highlighted text into a link (either internal or external to the site).
The star button is how you add images and videos to your site.
The source button shows you the underlying html being used to format your text. Note that this is editable, so if you know html you can make changes in here and it will take effect if the html is valid and allowed. Only a subset of html is allowed, for example style is not allowed.