LESS is an extension of CSS language simplifying and enhancing most common development tasks. LESS saves time and effort of UI developers, so they can concentrate on solving real problems, instead of battling trivialities over and over again.
The main difference between LESS and SASS is the former's CSS-like syntaxis so familiar to most developers. It only takes a few hours to learn a few basic principles to start writing code in LESS. Statistically LESS is a bit more popular than its counterpart, although there is no solid data to support this.