rootkit : Java Glossary

A rootkit is a clandestine type of malware that hides itself within operating system kernels or application binaries. Rootkits present more of a threat than ordinary viruses which attack garden variety applications because they make subtle changes to systems to open vulnerabilities and they cover their tracks. Rootkits are also extremely hard to detect and remain invisible to most of the security software we all depend upon. The problem is, once software is corrupt, the rootkit can defend itself against detection. You may never get rid of it without formatting and reinstalling all executables from scratch.