CSS means cascading style sheet. You use CSS to set the background color, font color, font style etc. of the contents of a webpage. In line css is when you change one line of code by adding to it's start tag. Internal css is when you set the style of each line in the head of the html. External css is when you set the style of the different parts of the page on a seperate file and you link that file in the head of the html.