What it is.
Malware is short for malicious software. For is practice it would have viruses, Trojan horses and spyware.
Viruses, Trojan horses and spyware
Viruses copies it self and need contact to spread. trojan horses seems legit but allows acces to someone else. and spyware secretly obtains personal info
How to prevent it.
antivirus software, firewall and common sense.