Rootkits are a variety of malware, a sort of virus, which first appeared in the Unix/Linux environment, and more recently in Windows.
Rootkits can be difficult to detect and sometimes difficult to eradicate. This is because they have two original features:
- firstly, they deeply modify how the operating system works (possibly at kernel level);
- secondly, they hide themselves from the operating system.
They can even hide various other malicious software that they have installed: spyware, backdoors, Trojan horses etc., this being their main reason for existence.