[caf]

Right. a "rootkit" was a kit of tools you deploy once you have obtained root (on someone else's server).

[cmdrfred]

I always thought of them as a way to gain root. As in privilege escalation.

[detaro]

A rootkit might come with tools for that, but the actual rootkit generally requires you having root (or some other privileged role) to deploy it. E.g. a Linux rootkit commonly is a kernel module, which you can only load if you have already obtained root privileges.