Mashups consists of taking dances from other songs and mixing them together to create a new dance. Usually, especially in the earlier games, the dancer(s) from the classic version of the song would appear in the mashup as well, but there have been times where a dancer from an alternate version appears or, more commonly in the later games, have no dancers from the song appear in the mashup whatsoever. Mashups commonly use solo dances, but the game has done mashups containing duet dances, trio dances, and dance crew dances.
Mashups are unlocked through Mojo. In Just Dance 3 and Just Dance 4, they are unlocked when the player completes a level in the mojo container (the latter has the player going through the Wheel of Gifts, which sometimes has a mashup as one of the available prizes) while in Just Dance 2014 and the games afterward, they are unlocked with Mojo Coins.
Mashups from Just Dance 2015 and beyond have themes. As such, they contain dancers that match the theme or are close enough to fit the theme. Prior to these themes, there were a few mashups in Just Dance 3 that were labeled as Sweat Mashups, those requiring more energy when playing them.
The settings for most mashups for Just Dance 3 had holographs of the current dancer dancing with a colored background. The Sweat Mashups on the other hand would take place in a gym, the same setting used for the sweat version of I Was Made For Lovin' You.
The mashups for Just Dance 4 would take place on a set with a bunch of circles placed on it. This background is also used for the extreme versions of Good Feeling and Run The Show and all of the Puppet Masters.
For Just Dance 2014, the setting used for the mashups had holograms of past dancers in the series placed all around a big disco ball in the middle. Whenever a dancer would come in, a hologram would move to the center and turn into a dancer; the holograph would normally not correspond with the actual dancer though. The setting also had a tendency to switch colors.
Starting in Just Dance 2015, as well as the Just Dance Unlimited service, the setting would take place in are similar to the ones in Just Dance 3. The only difference is that they would change auras.
For a list of songs that have a mashup see Songs with a Mashup.