A ramdisk is what iTunes boots to upgrade the firmware and the NOR, and in the case of the iPhone and 3G iPad, it upgrades the baseband.
It is signed, but if you have pwned the device with a bootrom exploit, you can load modified ones.
There are two types of ramdisks. They are very similar, and both are located in the IPSW. There is a Restore Ramdisk and an Update Ramdisk. To decrypt the ramdisks, follow the instructions on the Decrypting Firmwares page.