Diablo is your classic hack-n-slash RPG which has started a legacy of similar looking games. It is simple enough to get into. Your task is to beat Diablo but you'll need to get past his minions first. The best part about Diablo is its online feature which allows you to take on Diablo with other players around the world.
The problem with this game is that it gets quite monotonous after a while. Yes, fighting is enjoyable and fun but there's only so much you can do. It's not a RPG, per se, so hard-core RPGers need not apply. It's more an action adventure as it has limited RPG elements. Yes yes, there are stats which you can increase, quests which you can do, weapons which get better etc. but it lacks the feel of a true RPG which often have very engaging plots and the ability to do a lot more than just run head-first into battle.
Still, Diablo is a classic game. It is highly enjoyable since it is quite easy to pick up. Be warned though, it can also be rather addictive if you can get past the "non-RPG-type" elements!