CSS, or Cascading Style Sheets, is a style sheet language developed to control the presentation of markup language documents, like HTML. You can think of HTML as controlling the structure of the web while CSS controls the presentation of it.
CSS styles are applied in the order that they're found and since styles can be placed in several different locations, this often results in a cascade of styles, from external documents all the way down to locally placed styles.
You can think of web sites as being controlled and constructed by five main elements:
- HTML controls the document structure;
- CSS controls the documents' look and field;
- server-side languages such as PHP control processing and business logic;
- and databases such as MySQL store content.