External CSS
What this means in regards to HTML is that there is an external reference to the rules for the website. For example it’s one thing for you to remember rules for work based on what you’ve been told verbally but there’s also an external rule book that if changed will dictate future workings of your day to day life. Same case when it comes to a website. We can tell the website all we want how to act but if we give it an external rule book to follow, then any changes we make in the rule book the site will comply.
Something to remember with this ruling though, is that external sheets should not contain any HTML tags. It’s simply to signify certain characters and what they should follow.
Internal CSS
Similar to the External CSS there’s an Internal CSS however the difference with this one is it’s set up to give individual pages a unique set of stylized rules. To signify that a sheet should have a designated style *
Inline CSS
Finally we have an inline css, compared to the other two this one is designed to give a unique style to a single element rather than page(s) as a whole. Similar to the others you can assign them to paragraphs or other parts of the website however setting this rule would be done inside the body of the website rather than on the outside of what was occuring. Doing this assigns the style to that specific instance of the element instead of designating that all instances of the element will follow the same ruling.