I'm shopping for new pans right now and keep seeing cast iron advertized as "preseasoned". What does this mean? What are they preseasoning it with?
To add to Jay's answer:
The oil that is used to preseason cast iron undoubtedly varies.
Looking at Lodge's website I found the following:
"The cast iron is sprayed with a soy-based vegetable oil and then baked on at a very high temperature. The heat allows the oil to penetrate into the iron, creating a black patina finish with superior appearance and performance than home seasoning."
Cast iron inherently rust easily and does not provide a smooth nonstick surface to cook on. Seasoning cast iron provides a protective layer that will protect it from rusting and give the cast iron a nonstick surface. This is the way people got nonstick cooking ware before teflon and its directives or invented.
The seasoning on cast iron is formed by fat polymerization. Fat polymerization is maximized with a drying oil. This is usually achieved by treating the cast iron with a oil and then heating it.
Preseasoned means that cast iron does not need to be treated right out of the "bag". However note that the seasoning does not last forever. There are 100s of sites on seasoning cast iron if you google "seasoning cast iron."
To add to the additional question that the OP added, it is usually treated with vegetable oil, animal fat(lard), or according to this website, it recommends flaxseed oil which is a "drying oil."
It does matter what oil you use? Here are some hints: What oil do artists mix with pigment for a high quality oil paint that dries hard and glassy on the canvas? What oil is commonly used by woodturners to give their sculptures a protective, soft-sheen finish? It’s the same oil. Now what is the food-grade equivalent of this oil?
The oil used by artists and woodturners is linseed oil. The food-grade equivalent is called flaxseed oil. This oil is ideal for seasoning cast iron for the same reason it’s an ideal base for oil paint and wood finishes. It’s a “drying oil”, which means it can transform into a hard, tough film.