The Cascading Style Sheets Specification (CSS) is a computer language that is used to write formatting instructions (rules). These rules tell a web browser how webpage content should ‘look’— in terms of:

position, alignment, width, height, etc.
typeface, font-weight, colour, border, etc.

As an example, all level-one headings can be set in the typeface Verdana and coloured red with the rule: