Virus - Spreads by attaching itself to something and requires human interaction to replicate. Usually can mutate in attempt to avoid detection.

Worm - Can self-replicate. Can exploit networks to replicate quickly. Email worms most common - spread as file attachments which the user is tricked into running, or embedded in HTML email or just as a direct link to malicious code.

Trojan - Hides within another container, but can't self-replicate. Often used as the first stage of an attack (eg. used to download and install a stronger threat like a backdoor to your PC - known as remote access trojan or RAT)

Blended Threats - Combine worst aspects of the above - stealth, replication and payload. Using server and internet application vulnerabilities, can self-replicate quickly and cause great damage.

Drive-by Download - Injection of malicious code into otherwise innocuous web pages. Typically, the hacker adds iFrames to the page which invisibly load content which tries to exploit browser vulnerabilities in order to infect the victim's PC.

Rootkit - Modifies parts of the operating system or installed as drivers or kernel modules. They are a method of hiding running processes.

Spyware - Software that collects personal data without consent.

Phishing - Using a couple of simple tricks to send users to what they believe is a genuine site but is in fact a clone set up to steal their login details