CSS means Cascading Style Sheets language and its latest version is CSS3. It's showing how HTML element should be display exactly. It has that kind of things which is we can say mandatory for any site. It has decorative stuffs for skeleton of site. Its last version was CSS2 which was old school now and current version is CSS3.
Comparing to last version with latest version of CSS is totally changed. The latest version has Animation, Transition, Transformation, Gradients, and Shadows for Objects and has new kind of layouts like multi-columns, Grid Layouts and Flex Layouts.
The CSS language has many type of attributes like Fonts, Box, Elements, Margin, Padding, Icons, Links, Table Structure, Positions, Alignment of Elements, Combinations, Images and Controlling elements etc..
It has also 2D and 3D Transformation, Animations for any elements, Tooltips, Images related styles and most important is Media Queries which is very helpful for any site to match with current trend.
So, this is small information about the CSS but there are many things which are useful while using or working with CSS and which make the site more attractive and readable.