Both SASS and LESS are CSS preprocessors, which allow writing clean CSS in a programming construct instead of static rules.
As of Less 1.4, Less supports nested, inherited rules via the &:extends and @extends pseudo-selector. Prior to this, a main difference between Less and other preprocessors like Sass was the lack of an @extends directive to support inheritance of rules across classes, leading to cleaner CSS with less duplication.
LESS is inspired by Sass. Sass was designed to both simplify and extend CSS, so things like curly braces were removed from the syntax. Less was designed to be as close to CSS as possible, so the syntax is identical to existing CSS code. As a result, existing CSS can be used as valid Less code.
The newer versions of Sass also introduced a CSS-like syntax called SCSS (Sassy CSS)