ANSWER:
A rootkit, also spelled root kit, is best defined as a set of
programs which
enables administrative (root level) access on a computer or
a network of computers.
Typically,
rootkits use various methods which allow them to be very hidden
from the end user of the computer. Rootkits normally modify
the code of at least one system file therefore becoming part
of the system. These are often very powerful programs which
can have a nearly unlimited amount of control of the computer
that they exist on. Rootkits also commonly hide the existence
of files or programs that are running on the computer. Rootkits
often are used to hide viruses
or other forms of malware.
There
are occasionally "legitimate" reasons for a rootkit,
though it is my opinion is that any program that does not forewarn
you that it will install itself as a rootkit should never be
considered desirable. If a company feels the need to hide files
and processes from the end user without notice, why should the
end user trust this company?