Cascading Style Sheets (hereafter CSS) are used to format the content of web pages. The W3C is working on v3 of CSS with 2.1 being the most recent (1998) aurhorized specification. Each version of CSS (so far) has been designed to
The idea behind CSS is to separate the formatting definitions from the document being formatted. This allows the definitions of various display styles used over several documents to be stored in a single document. This makes the appearance of multiple pages within a site far more uniform. The point of CSS being cascading is to set a hierarchy of inheritance of style information. Style information within a format block within a document overrides any style information in the header of the document. External style definitions are overridden by any styles anywhere in the document itself.
This idea of inheritance allows the style to be applied to anything that doesn't specifically have it's own style formatting while still allowing the web page designer to override the style sheet for specific exceptions.