What is a backdoor?

More correctly called a Remote Access Trojan, or RAT, a backdoor is a type of malware that allows unauthorized access to a computer.  A RAT can give the hacker complete control of the system.  He can install other malware or explore the PC, looking for password lists with logins to bank accounts or tax returns to gain information for identity theft.  The hacker can also use the infected computer as a staging area for attacks on other computers or servers without the knowledge of the infected computer’s owner.  Computers under the control of the hacker are called “zombies.”  (See also, What is a botnet?)