Those are 3 TV systems.
PAL is used in Europe , Australia & some Asian/African countries.
NTSC/U is used America
NTSC/J is used in Japan and some Asian countries.
This means an european Xbox , also called PAL xbox , will output a "pal-stream". An american xbox will output a "NTSC/U-stream".
Most new TV's can handle the 3 systems thought.
PAL,NTSC/U,NTSC/J is also used as territories. PAL games will only work on a PAL xbox , NTSC/U games will only work on a NTSC/U xbox , ...