The Cave is set up like an old-school 2D side-scroller. Seven adventurers, each with a dark past, go down into a magical cave in a journey of self discovery. At the beginning of the game (which is narrated throughout by The Cave itself), you choose three of the seven advernturers and descend. Each section of The Cave is a puzzle that requires your three characters to work together. Three of the sections are uniquely designed (by the sentient Cave) for the characters you've chosen. In this way, The Cave encourages you to play though multiple times.
The drawback is that there's also three generic puzzle sections which you cannot skip. I'm on my second playthrough, and while it's nice that the game shuffles the order of the puzzles, it can be a drag to re-solve old puzzles to get to new content. And the achievements require you to play through the game... I don't know how many times, but I imagine The Cave's jokes (while clever) will start to wear thin on the fifth playthrough. If you really want to get 100% achievements, you might consider reviewing the achievement list online before you play, despite the mild spoilers.
I'd still recommend this game - the humor, art and story make this puzzle game very enjoyable and encourage multiple playthroughs. May get monotonous if you're looking for all the achievements.
A definite buy if it's on sale.