A CSS style sheet is otherwise known as a Cascading Style Sheet. When first building a website, it can be hard to understand all of the HTML mark-up language. CSS allows this language to be seperate from the html document and contains all of the mark-up language. It enables the seperation of the content that's in the document from the document design. Style sheets are commonly saved as .css.
An internal stylesheet is used when a single document has it's own distinct style. You apply this in the heading section of the HTML page. An external stylesheet is essential when you have a unique style that you want to use for the entire web site. You can change the display of an entire website by just altering one file.