Allows you to mount iSCSI exports from some server (I use iscsitarget - http://iscsitarget.sourceforge.net - on my Linux server).
It consists of a kernel extension and a userspace control app.
iSCSI is a way to access disk space over TCP/IP. What you get is a raw block device, so you can put any file system on it - indeed, you can use it for Time Machine.