A back door is a method of access into a computer software. A user normally uses back door for the purpose of troubleshooting or to apply patches to the software, without going trough the length security procedures, but this is a major security risk.
The word speaks for it self, it is a access point which has low or no security at all, which becomes a major security risk, since crackers can easily exploit the software and bypass all the security mechanism, while trying to remain undetected. Crackers also develop worms which are designed to take advantage of a back door created by an earlier attacker.
A back door in computer field is exactly the same as the back door of your house, it is a easy access and great for renaming undetected, but to avoid from any unknown member to penetrate it, it is better to keep it secured.