Malware is malicious software that can be used to obtain your sensitive information or delete or modify files. The most common types of malware are Viruses,Worms, Adware,Spyware,Trojans, Root Kits, Backdoors, Ransomware and Botnets.
Backdoors are most commonly installed after an attacker has gain access to your system and wants to maintain that access. If you discovered your system has been compromised, power down your computer and contact a cyber security professional.