The mast camera is on curiosity but I don't really know what its about . it is know that it's official on curiosity but I don't know what it's used for.
The mast on Curiosity is used to place cameras and other instruments far higher than the top deck of the rover body. Other rovers use a similar mast. This gives these instruments a better field of view.
Mastcam is used to provide scientific images (for geology), context images for the other instruments, and to help with navigation.
The Mast Camera, or Mastcam for short, takes color images and color video footage of the Martian terrain. The images can be stitched together to create panoramas of the landscape around the rover.
The mast contains more cameras: Navcams for navigation (4x) and Chemcam, which is used in combination with a powerful laser for spectroscopic analysis of the soil.
The Mastcams (there are 2) on the Mars Science Laboratory aka Curiosity were built by Malin Space Science Systems and use similar components to the other imagers on Curiosity such as MAHLI and MARDI. They are developments and upgrades of similar cameras that were flown on Spirit and Opportunity, the previous "generation" of Mars rovers.
There are more details on the Mastcams on the specific page at MSSS but basically there is a short focal length (34mm) Mastcam, giving a wider field of view (15$^\circ$), and a longer 100m focal length Mastcam for more close-up images (5.1$^\circ$ field of view). Both are equipped with several filters in the visible and near-infrared part of the spectrum (the filters span 440 to 1035nm) to allow the mineralogy of rocks and the terrain to be determined but also to allow natural color images to be taken. In addition both Mastcams are capable of 720p video (at 10 frames/second) and in theory can be used for making 3d stereo pairs (although this is apparently not often used due to the different focal lengths and fields of view of the 2 cameras).