Some rootkits install a backdoor. Not all rootkits install a backdoor -- some merely conceal themselves and operate locally. The famous Sony Rootkit is one such example of a rootkit which did not add a backdoor.
The defining characteristic of a rootkit is that it conceals its presence from the rest of the system. Backdoor.OSX.Mokes.a doesn't really do this -- it's only a backdoor. Not a rootkit.
Unsure how this clears up the rootkit versus backdoor confusion...
The sony rootkit was named somewhat incorrectly, because it also tried to hide itself and no other existing malware names fit it.
Are you sure? It also commonly referred to such kits being used by hostile parties. I've personally interrupted an attempt at installing the "Hungarian Rootkit" in the 90's. (I put unpatched Red Hat 6 online when Red Hat 7 was out.)
(that's where the name comes from root = admin on unix)
The fact that you think this is something that bears explaining is interesting in the context of HN. I hope this is based on something you've noticed about recent user trends here. There was a time when someone would be very surprised if a user here didn't already know this.
Rootkits are the reason why it is recommended to wipe the whole system after being hacked, because you can't be sure there there wasn't anything installed.
I suspect that's exactly what he means - a rootkit is deployed by an intruder so that when the admin discovers the host has been compromised and patches the vulnerability, the rootkit, if not addressed, will grant the intruder root capabilities once more.
