What’s a rootkit, and how can I check for rootkits installed on my machine?

A rootkit is a term used to describe mechanisms that allow malware such as viruses and spyware to hide their existence from tools that are designed to eradicate them. Rootkits commonly open back doors to systems so that malicious intruders can access the system with administrative credentials or intruders use them on the machine to maintain their access. See here for more information about rootkits.