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On what device? 7200?
Bootflash generally hosts a bootloader program. Depending on device, it might be a very small basic bootloader, i.e. just enough to perhaps do some quick hardware startup diagnostics and then boots the main IOS. Or, bootlflash might contain a "miniature" IOS. Flash generally hosts the main IOS program.
When there's a bootflash, it's sometimes the destination of crash files.
Some Cisco devices have a bootrom that is erasable. While a bootflash can often be worked with using typical flash commands (e.g. dir, format, copy, del) and has a directory structure, flashable bootrom normally requires a special command to update it and doesn't have a directory structure.
Bootflash, like most earlier flash memory, is normally provided by internal flash memory.