What is "less"
Less (also known as LESS) is a pre-processor of CSS style sheets, open-source, and extends the CSS with dynamic behaviors as variables, mixins, operations and functions. It was developed by Alexis Sellier, known as cloudhead, and the code is hosted on Github.
Less is written in Javascript, and can run on either the client side or the server side (with Node.js, Rhino, etc.). The code compiles the Less syntax for CSS, so what the browser renders is CSS, not Less. The syntax is similar to CSS, and you can mix common CSS and Less in the same file, which leads to extension .less.
Links:
Compilers Less Online: