ummm lets see if i can explain it well enough so that it can be understood without too much complexity.
A rootkit, is exactly what it is called. "Root" being the highest level Administrator on any pc. Primarily "root" is used in the linux/unix world, however same holds true on a windows box. a rootkit is designed for the looser who is using it, to somehow or another gain "root" access to the targeted system. it works by infecting all running processes and turning them into slaves to keep it simple. From that point they can do many things. spam, download and run trojans, or any other app designed to.
not sure if that answered the question or not. but let me know if not.
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to add to this.
they are not detected as a virus, because they are not. they are much more complex than a "normal" virus. and most of them run, do what they are ment to do, then erase their tracks and delete themselves so that they cannot be detected as easily. some antispyware software can now detect them, however they are not fool proof. most rootkits are very "advanced"