CSS preprocessors e.g LESS

CSS is a powerful lanaguage but over time users have found it lacks features and can easily become very complex especially on larger sites. CSS preprocessers like LESS, SASS, STYlUS etc evolved to fill these gaps in functionality and make css writing more efficient.

A CSS Processor - is a two step process, it extends css via a scripting language and then compiles that syntax into regular css once it is run through a processor.