Puff pastry is a rich, multilayered dough. This traditional French pastry is known as a "laminated" dough because it alternates layers of fat (usually butter) with dough. It's used to make a variety of classic French pastries like croissants, napoleons, and palmiers.
Puff pastry is made by placing a block of chilled fat between layers of pastry dough, then rolling it out, folding it in thirds and letting it rest. This process, usually repeated 6 to 8 times, produces hundreds of layers of dough and butter. When baked, the butter melts creating gaps between the dough layers, the water in the dough and in the butter turn to steam, filling the the gaps and forcing the dough to puff and separate into hundreds of flaky layers. Because the process is labor-intensive (and because frozen puff pastry is delicious and simple), most home cooks don't make their own. But if you do, the results are well worth the effort.
Puff pastry is not to be mistaken for phyllo dough, which is also commonly sold frozen. Both are flaky and crisp, but puff pastry is incredibly buttery, soft, and of course, puffy, where phyllo dough is not. The two are generally not interchangeable.