Differences between SCSS and LESS

SCSS and LESS are both "supersets" of vanilla CSS. That means valid CSS is automatically also valid SCSS and LESS, but not necessarily the other way around.

The difference between SCSS and LESS is that they have a different syntax. They mostly have the same functionalities.

You can read more about SCSS and LESS here.

Both Sass and Less are CSS preprocessors.

From keycdn.com

A CSS preprocessor is basically a scripting language that extends CSS and then compiles it into regular CSS.

So Sass and Less don't change the functionality of CSS, as they are compiled into plain old CSS. What they do is make it easier to write and maintain CSS with tools such as mixins, nesting, variables, and more.

SCSS, which stands for 'Sassy CSS' is one of two syntaxes for Sass.
From the sass reference:

The second and older syntax, known as the indented syntax (or sometimes just "Sass"), provides a more concise way of writing CSS.

The difference

Both Sass and Less have very similar features. Sass uses Ruby whereas Less uses Javascript. There are syntactical differences, for example, Sass uses $ for variables whereas less uses @.
There are some slightly more subjective differences, this website claims that "LESS Has More User-Friendly Documentation Than Sass", however personally I have found the Sass documentation and examples very easy to use.