What is tethered and untethered?

If a jailbreak is tethered, you have to connect to the computer once you reboot to rejailbreak.
If a jailbreak is untetherd, you can reboot without having to connect to a computer.

Current jailbreaks:

4.1:
redsn0w, untethered
sn0wbreeze, untethered
Pwnage Tool, untethered

4.0/4.0.1:
redsn0w, untethered
Star (jailbreakme.com), untethered
sn0wbreeze, untethered
Pwnage Tool, untethered
iBooty (sn0wbreeze, Beta only), tethered

3.1.3:
redsn0w, untethered
sn0wbreeze, untethered
Pwnage Tool, untethered
Spirit, untethered

3.1.2:
redsn0w, untethered
sn0wbreeze, untethered
Pwnage Tool, untethered
blackra1n, tethered