What's the difference between CSS Snippet and Code Snippet in my settings?

Both come with the Code Snippets plugin:

• CSS Snippet: three boxes of CSS. All devices applies everywhere, Tablet only on screens up to 1024px wide, and Mobile only up to 640px. Phones also fall under the tablet width, so tablet rules apply there too. Click Save Changes.
• Code Snippet: separate named snippets of HTML / JS, CSS or JavaScript, each with a Location, Show on pages, Priority and an Enabled switch.

For a quick style fix, CSS Snippet is simplest. Use Code Snippet when you need scripts, page targeting, or several snippets you can turn on and off separately.