Malware
Malicious software
Viruses
Copies itself; Replicates by inserting copies of itself into other programs. Viruses need contact to spread.
Trojan Horses
Seems legit BUT allows access to someone else. Trojan horses allows others to get information from your computer.
Spyware
Secretly obtains the users information
How To Avoid Them
You can use...
AntiVirus Software
Firewall
&
Common Sense!