Katsu is short for KATSURETSU, which is the Japanese way of pronouncing cutlets.
Don is short for DONBURI., which means BOWL in Japanese.
Katsudon is KATSU over a bowl of rice.
It is quite heavy and is very popular among children and younger group in their 20's and 30's.
History about KATSU(video):
How to make Katsudon(video recipe):