I've read about a "pancake lens" but don't really understand what that means.
What the pros and cons of having a pancake vs. another type of lens?
I'm thinking of getting a Panasonic Lumix DMC-GF1 which comes with such a lens.
A pancake lens is designed to be physically very thin and compact, being relatively pocketable when combined with a slim body.
Current pancake lenses have a fixed focal length in the middle of the spectrum (neither wide nor telephoto), and they are relatively slow for a prime lens (often f/2.8) although exceptions exist. The image from a pancake lens looks the same as an image from a bulkier lens of the same focal length; they have no special effect.
Although the pancake design is quite old, it has been popularized lately by mirrorless systems. Currently produced pancake lenses include:
Here is an example of a pancake lens:
"Pancake lens" is a purely physical description.
Actual overall size has nothing to do with it, and it has no implications about image performance, aside from the fact that it's generally very hard to pack a telephoto or fast prime into the pancake form-factor.
It's worth noting that a "Pancake lens" does not have to be small or compact, and a small lens is not automatically a pancake lens. It's only a description of the lens form-factor.