Backdoor
What is a backdoor
A backdoor is a silent way into your computer via a website, software, or operating system. It is achieved by exploiting a vulnerability in one of them to get in and control your computer without your knowledge.
Why are they bad
Backdoors are bad as they compromise the safety of your computer, your network, your files, and anything else related to it. They can launch botnets which silently run on your computer without your knowledge for illegal use.
How can I protect myself from backdoors
Update your software to its current version.
Do not use unlicensed (hacked/cracked) software.
Use, and have an up to date antivirus software.
Keep your Operating System up to date.