Backdoors are security risks that allow attackers to remotely and stealthily access a computer using either plain-text or fully visual interfaces.
Such backdoors are either opened by a Trojan (for instance, the much-hyped Back Orifice remote access tool), or even opened by an existing piece of software on the computer.
Moreover, backdoor features may also be included in commercial software by using a hardcoded combination of username and password. Many times, backdoors are deliberately left open by computer vendors. However, please note that this kind of backdoor is not to be regarded as malware, as it is mostly used for support and maintenance tasks (support staff usually connects to the respective machine remotely, in order to troubleshoot various operation issues).
Still, such backdoors can pose a security risk to the end-user, as they can be easily exploited by unauthorized parties. Unlike spam and Trojans, backdoors don