Backdoors are most often used for securing remote access to a computer, or obtaining access to plaintext in cryptographic systems. From there it may be used to gain access to privileged information like passwords, corrupt or delete data on hard drives, or transfer information within autoschediastic networks.
A malicious backdoor is code that allows unauthorized and often unrestricted access to a compromised site. They allow attackers access to all of the files within the hosting account. Backdoors can look like normal php code or obfuscated (intentionally obscured to make code ambiguous) and hidden.