CSS är sammlingsnamnet för CSS alternativt menar man CSS level 1, säger man CSS 3 menar man CSS level 3.
Det är alltså rekomendationer (=webstandarder) från w3c.
Kolla länkarna för mera info om skillnaderna.
http://www.w3.org/Style/CSS/
http://en.wikipedia.org/wiki/Cascading_Style_Sheets
http://en.wikipedia.org/wiki/Cascading_Style_Sheets#Variations
"Cascading Style Sheets (CSS) is a style sheet language used to describe the presentation semantics (the look and formatting) of a document written in a markup language. Its most common application is to style web pages written in HTML and XHTML, but the language can also be applied to any kind of XML document, including SVG and XUL."