An audio mashup takes parts of two or more recorded songs by different artists – maybe people who would never willingly be part of one another’s music – and creates a new song based entirely on these source songs.
Mashups were originally done mostly by amateurs using computer software to combine the elements of the songs.
Video mashup is combining elements from various video and audio sources to make an “original” video. People have come to use mashup in other extended senses: for instance, web pages that include widgets from Google and are thus “Google mashups” – any practice that creates something new by combining previously created stuff.