What is a "mash-up"?

A mash-up is commonly used to describe a web page, song or other piece of media that uses previous pieces of  media and combines them into one song. Song versions are very popular on YouTube as they are funny, such as 'Set Fire To Yourself', a mash-up of Eminem's song Lose Yourself and Adele's song Set Fire To The Rain. With songs, they are often protected by the 'Fair Use' section of the Copyright laws. This means that they are changing the content, not simply posting it as their own without changing it.