udev is the replacement for the LinuxKernel's devfs.
DevFs was a method for the kernel to create entries in /dev for available devices when modules were loaded. devfs is now obsolete.
udev is entirely in user-space, using HotPlug and the /sys filesystem. udev is only in kernel 2.6 and above.
See 
http://www.redhat.com/magazine/002dec04/features/udev/ for a good overview.
Q: Why dosn't the HAL and UDEV combination pick up my device events (ie plugging in a usbdrive) in Debian?
A: You have to add the user to the "plugdev" group, else it won't work.
10 pages link to UDev:
