A backdoor in a computer system or crypto algorithm is a covert attempt to circumvent normal authentication measures or other processes by allowing for secure remote access (or access to plaintext in the case of a crypto algorithm).
Backdoors on computer systems take the form of a feature of installed programs or rootkits, and can become a real threat with the rise of multi-user networked operating systems and computers. Botnet owners are able to launch distributed denial-of-service (DDoS) attacks using their bots as a result of the backdoor installed on their bot machines.