It is usually an amber ale with very mellow hop flavor and an addition of cinnamon, all spice and nutmeg. The largest difference you will find between them are the amount of spices used. I tend to lean towards the ones that have a little less spice flavor in them as it can be overpowering pretty quickly.
DogfishHead has a pretty good one, but the spices can be a little overpowering sometimes.
Some pumpkin ales bypass the use of pumpkin in their pumpkin ales, but I don't like those nearly as much. I use fresh pumpkin in mine.