[MEncoder-users] 12bpp, what does it mean?

[Pepe]

I realized that all the videos I created with mencoder say something like this 
when playing with mplayer:

VIDEO:  [XVID]  576x432  12bpp  25.000 fps  824.2 kbps (100.6 kbyte/s)

But videos I downloaded from internet say 24bpp instead of 12bpp. What does 
12bpp mean? 

Is it because my screen is configured to 16 bit color? But what does it have 
to do for creating a video?

-- 
Pepe