A backdoor is the act of providing hidden access, both software and hardware, to a computer network or machine in a malicious manner without the knowledge of the legitimate user of the system.
Most backdoors are malicious software that needs to be installed on a computer in some way. However, some backdoors do not need to be installed because some parts are already built into software that works with the remote host. Sometimes programmers leave these backdoors in their software for diagnostic and troubleshooting purposes. But hackers use them to break into a system.
Unlike other types of virus/malware, backdoor attack elements reach into the core of the targeted application and often control the targeted resource as a key driver or administrator. When access is gained to such a deep and critical level, the possibilities for damage are endless. Attackers can modify all or part of the infrastructure, make the target system run/port as they wish, and steal critical data.