It's the daemon that mounts your SD card so that you can use it.
It also mounts /media and is responsible for switching to USB mode when you plug the Nook into a desktop.
It's also what should be mounting your USB flash drives when you plug them in.
While working on my adbsync.exe I kept on running into bugs related to the vfat file system on the SD card.
Using the adage of "It's better to fix someone else's bugs than your own bugs", I set to work on vold.
The vold that ships with the Nook is non-standard.
I don't know if it was snarfed by B&N from somewhere.
It won't mount ext3 volumes or USB flash drives.
I built vold from the Eclair sources and it works even worse than the B&N stock.
A bunch of hacking later, I got it to mount an ext3 partition on an SD card.
I'm still working on USB drives and multiple partitions on an SD card.
So my adbsync.exe works marvelously now that it has real UTC timestamps to go on (courtesy of ext3).
As soon as I get things a bit more stabilized I'll post vold.