Exploits are programs that contain data or executable code which take advantage of one or more vulnerabilities in software running on a local or remote computer for clearly malicious purposes.
Often, malicious users employ an exploit to penetrate a victim computer in order to subsequently install malicious code (for example, to infect all visitors to a compromised website with a malicious program). Additionally, exploits are commonly used by Net-Worms in order to hack a victim computer without any action being required from the user.
Nuker programs are notable among exploits; such programs send specially crafted requests to local or remote computers, causing the system to crash.