aps aps c?

Can somone explain what the difference is between APS and C APS like the 20d??/???

APS = Advanced Photo System. It was a new system, invented for photo cameras using film, as an alternative to 35mm film.

From Wikipedia: The film is 24 mm wide, and has three image formats:

H for "HDTV" (30.2 x 16.7 mm; aspect ratio 16:9; 4x7" print)
C for "classic" (25.1 x 16.7 mm; aspect ratio 3:2; 4x6" print)
P for "panoramic" (30.2 x 9.5 mm; aspect ratio 3:1; 4x12" print)

The sensor of the 20D is approximately the same size as the APS-C format (22.5 x 15mm). Note, the sensor size is the only resemblance between the 20D and APS(-C), anything else that's said about APS does not apply to the 20D.