I would define a dungeon crawl as an RPG focusing on digging through one large dungeon, or several large dungeons, rather than an expansive world. Gameplay can vary greatly between them, but that's the major thing that really unites them all.  I'd say as far as classic games go, the ones that personify it best would be Etrian Odyssey, Diablo, and Eye of the Beholder. You should care because these games are often challenging and deep, with fun mechanics to experiment with. They don't tend to focus on story, and thus they avoid many of the pitfalls of many modern RPGs that do. As far as what games to try, I would start with Etrian Odyssey. It is quite challenging, but it avoids the many interface and game system issues I've seen with Wizardry clones. You will have trouble advancing in the game, but it won't be because you get lost or had no idea what stats to set to gain access to certain classes. Next up, I'd give Eye of the Beholder a go. It uses real-time combat, which can be tough, but it has a solid implementation of the Second Edition Dungeons and Dragons ruleset. The dungeon is lengthy and full of secrets to find. Finally, give Dungeon Siege 3 a try. It changes up various action RPG conventions that we've been stuck with,  making the combat fresh and engaging. You could also play some roguelikes, but those are certainly not for everyone!