CSS3 is the latest standard for CSS.
Cascading Style Sheets (CSS) is a simple mechanism for adding style (e.g., fonts, colors, spacing) to Web documents.
CSS has a simple syntax and uses a number of English keywords to specify the names of various style properties.
A style sheet consists of a list of rules. Each rule or rule-set consists of one or more selectors, and a declaration block.
In simple terms CSS is what allows programmers change the look of a website.
The CSS Working Group began tackling issues that had not been addressed with CSS level 1, resulting in the creation of CSS level 2 on November 4, 1997. It was published as a W3C Recommendation on May 12, 1998. CSS level 3, which was started in 1998, is still under development as of 2014.
Maybe the biggest difference between CSS2 and CSS3 is the separation of modules. While in the previous version everything was a large single specification defining different features, CSS3 is divided into several documents which are called modules.
The media queries might well be the most important addition to CSS. What it does is simple: it allows certain conditions to be applied to different stylesheets, making websites fluid and fit all kinds of screen sizes. Media queries allow developers to tailor to different resolutions without having to change or remove content.
Back when CSS3 was newly released, vendor prefixes were used all the time, as they helped browsers interpret the code. Sometimes you still need to use them today, but as browsers update their support for CSS3 prefixes aren't needed.
Pathway to animation
Now that CSS3 has started to ingrain itself properly in all the major browsers, the web will advance and this allows for great new things. Animation is growing on the web and will be in the coming years a new great addition to website. Currently there are better options than CSS3 animations which is why we use other tools but for small transitions CSS3 works perfectly.