Computer Malware

Malicious software, better known as malware, is usually spread through e-mail and over the Internet. It comes in many forms and is used to steal sensitive data like credit card numbers and passwords off of computers. Malware refers to any type of software that was created to cause damage to a computer.
Common examples of malware include viruses, worms, trojan horses, and spyware. Viruses, for example, can cause havoc on a computer's hard drive by deleting files or directory information. Spyware can gather data from a user's system without the user knowing it.
Types of Malware 
Malware is not a virus. In fact, it consists of viruses, worms, Trojan horses, adware, rootkits and many other nasty infections. Some of these programs are more of an annoyance than anything, while others pose a serious threat to your computer.
Viruses
The computer virus is the most infamous form of malware. It is a self replicating program that infects a system without authorization. A virus is often transmitted via email but can also be distributed through various storage mediums such as a flash drive. Once installed, it will execute itself, infect system files, and attempt to propagate to other systems.
Spyware
Spyware is a sneaky program that tracks and reports your computing activity without consent. While it isn't designed to inflict damage, spyware can terribly affect the performance of your computer over time. Spyware usually comes bundled with free software and automatically installs itself with the program you intended to use. Signs of spyware include sudden modifications to your web browser, redirects of your search attempts and the frequent displaying of pop-ups.
Worms
Similar to a computer virus, worms are infectious and self-replicating. However, Computer worms work with computer networks. The worm utilizes a computer network to send replicas of itself to connecting computes on that network. Computer worms can replicate in create volume and it poses a great threat to large computer networks. Computer worms can be removed using malware removal tools.