Backbone.js gives structure to web applications by providing models with key-value binding and custom events, collections with a rich API of enumerable functions, views with declarative event handling, and connects it all to your existing API over a RESTful JSON interface.
LESS extends CSS with dynamic behavior such as variables, mixins, operations and functions. LESS runs on both the server-side (with Node.js and Rhino) or client-side (modern browsers only).
Sass is an extension of CSS3, adding nested rules, variables, mixins, selector inheritance, and more. It’s translated to well-formatted, standard CSS using the command line tool or a web-framework plugin.