A skillet is the pan with slanted sides. To add to the confusion, this pan is also sometimes called a frypan or frying pan. The slanted sides make this pan perfect for stir-frying and quick cooking techniques where you're moving ingredients around a lot in the pan. It's also good for dishes like frittatas that are served straight from the pan.
The reason these pans get so confused is because they really can be used almost interchangeably. They're constructed from the same kinds of metals and are available in the same diameters, so they only really differ in the shape of the sides. Sure, one might be slightly better suited for a particular task, but that doesn't mean that the other pan can't do it.
If you're trying to decide which kind of pan to buy, we'd recommend going for the skillet first. You lose a little surface area, but you can buy a slightly larger size than you might otherwise to make up for this. We feel like the sloping sides of the skillet give us better access to the food and make this pan more versatile.
Do you have a preference for one pan over the other or think it makes a big difference which one you use? Let us know!
(Images: Williams Sonoma)