DisplayPort is a digital display interface developed by the VESA. The interface is primarily used to connect a video source to a display device such as a computer monitor, though it can also be used to carry audio, USB, and other forms of data.
The Mini DisplayPort, developed by Apple, is a miniaturized version of the DisplayPort audio-visual digital interface. The functionality and signals are identical.
Now, the graphics card you are referring to has 2 mDPs, probably because it was aimed for users who use multiple monitors for video editing/modeling purposes.