CSS, Cascading Style Sheets, is a style sheet language that was first proposed in 1994 by Håkon Wium Lie. The main purpose of CSS is to separate the style of a page from the structure of the page. This separation can improve content accessibilty and provides more control. In addition:
Css allows multiply pages to share the same styles.
Css allows for different types of styles and rendering methods.
Css allows for different styles depending upon the size or the device on which the page is viewed.
Internal / External
Although the main purpose is to separate the style from the structure of the page you can still include styles within the body of the html but as this is such a no-no I will not even include an example. You can also include the styles within the head of the document in the <style></style> tags but this defeats the purpose of using one style sheet for multiple pages.
The best is to create an external style sheet and place the link to it from the head of your document as shown in the example below: <link rel="stylesheet" href="style.css">
If you find any errors or have relevant information you would like to add, please let me know.