Sniffing means that someone is putting some sort of listening device onto your system. Kind of like someone tapping off your phone. Also known as keypress logging . For example a cracker could install a snooper or backdoor as a trojan. A classic trojan is a screen that looks like the login-prompt and asks you for password and then saves it to disk.
Some trojans are very destructive.
A Software backdoor is a program that has a hidden function that allows someone who knows about it to bypass security. An example of a Setup backdoor is to set security so that anyone can change the administrators password. Crackers installs backdoors once initial access is gained to be able to get in easily later even if their logins are lost. Guessing passwords . But not allways.