The boot image is a subset of the Cisco IOS Software that is used to download main Cisco IOS Software images to the router using TFTP in recovery situations. The user can interact with this image through a command line interface signified by the prompt Router(boot)#. Some platforms (Cisco 1600, 2500) have it in ROM; others (high-end routers) have it in bootflash. Depending on your platform, this image may be called xboot image, rxboot image, bootstrap image, or boot loader/helper image.
The boot image has limited capabilities. For example, it does not contain routing information.