For lack of a better term, I'll describe Monster Master as Magic for dummies. (The comments say that it's even more similar to Yu-Gi-Oh, but not being familiar with that, I can't comment.) And when I say "for dummies", I mean that War (the card game) is to real war as this game is to Magic. In fact, it's barely more sophisticated than War (the card game).
So, you have a deck, and every turn you draw one card and play some cards. You can not play more than one monster per turn, but you can play as many spells (which just do what they say) as you have and want to play. Monsters take a certain amount of time to summon (longer for more powerful monsters), and cannot attack during that time (though they can be attacked). To attack an enemy monster, just click on your own monster and click on the monster you want to attack, and then the most fun thing in the game happens: randomness! Each monster has an attack and a defense value, and a die is rolled from 0 to that value for each monster. Whoever is higher wins, and the other monster loses an amount of health equal to the difference. There's a reason that CCGs tend not to have randomness in them, and that's because there's already an amazing amount of randomness in the draws from the deck. Can you imagine how irritating Magic would be if, after crappy draws, you finally managed to get a Force of Nature out, and then some Llanowar Elves managed to kill it? Well, you can experience that feeling in Monster Master! I suppose that, since all monster combats are one-on-one, it's the simplest way to prevent the most powerful monsters from steamrolling everything, but it's still deeply unsatisfying and often frustrating.
The graphics are very basic -- the cards don't have anything special in their design; while there's a bit of character art, it's very small. There's no sounds or music at all. The AI is competent, but not great. The card selection is pretty limited -- there's about 20 different types of monsters, and about 30 different spells, so there's just not much opportunity for diversification.
Overall, this is a game which might be diverting for a couple of minutes, but there's just not enough strategy or tactics there -- it feels liks it's pretty much all luck, and the lack of polish doesn't really help this game, either.