A preprocessor is a tool that transforms one type of code into another type. In this tutorial, we will look into two popular preprocessors: one for HTML (called Haml) and the other one for CSS (called Sass). Haml and Sass are much more easier to write, and provide a few very useful features out-of-the-box which are not available in HTML or CSS. Thus, by writing in Haml or Sass, and then using a preprocessor to convert your code into HTML or CSS respectively, you can simplify your development process quite a lot.