1. Viruses
Viruses cannot spread without the help of us humans. They are like parasites because they need a host to attach themselves to. The host is usually a legitimate looking program or file. Once this program is launched, the virus is executed and infects other files on your computer. Viruses can be very destructive. They can do damage to your computer hardware, software and files. Viruses are spread through the sharing of files and are many times sent within emails via attachments.
2. Worms
A worm is a malicious program that can replicate itself onto other computers on a network. Unlike a virus, worms don't need a human to be able to spread and infect systems. Once it infects a system, it uses that system to send out other copies of itself to other random systems attempting to infect them.
3. Trojan Horse
A trojan horse is a malicious program that can be used to do silly things to a system like changing its desktop, mess with the user interface, and take control of your mouse. It can also be used for some serious things like accessing your data, erasing your files, stealing your passwords, and capturing your keystrokes.
4. Logic Bombs
Logic bombs are usually pieces of code that are programmed into a program that lie dormant until a certain time or until a user does a certain action which causes it to be executed. When it is triggered it performs a certain function that the program wasn't intended to do.
5. Bacteria
Bacteria make many copies of themselves and eventually end up taking up all of the computers recourses such as all of its processor power, memory and disk space. This results in the legitimate user losing access to those resources.
6. Blended Threats
Blended threats combine all of the characteristics of the above and use them along with system vulnerabilities to spread and infect machines.
