.........There's two thing you need to know:
Pokemon/Pocket Monsters: Flip RPGs on it back by having you capture and train Monsters, as well as pit them against each other, rather than just plain kill then yourselves.
Digimon/Digital Monsters: Basically Tamagotchis meets bug fights. Enough said.
Only thing they have in common is that they're both fighting monsters. While Pokemon is mostly RPG, Digimon are V-Pets at the very core.