Malware is an extension of the original threat to computer security from viruses.  The term Malware is short for malicious software, which is software designed specifically to damage or disrupt your computer system.  The term encompasses not only viruses, but new threats such as Spyware, Trojan horses, and worms.

In computer security, a "virus" is a self-replicating computer program that spreads by inserting copies of itself into other executable code or documents. A computer virus behaves in a way similar to a biological virus, which spreads by attaching itself to cells. Keeping with the analogy, the "attachment" of a virus into the program is termed as an "infection", and the infected file, or executable code, that is not part of a file, is called a "host". Viruses are one of the several types of malware.

While viruses can be intentionally destructive, most other viruses are fairly benevolent or merely annoying. Some viruses have a delayed payload, which is sometimes referred to as a "bomb". For example, a virus might display a message on a specific day or wait until it has infected a certain number of hosts. A "time bomb" occurs during a particular date or time, and a "logic bomb" occurs when the user of a computer takes an action that triggers the bomb. The predominant negative effect of viruses is their uncontrolled self-reproduction, which wastes or overwhelms computer resources.  As a result your computer will run slower, give you random errors, programs will not open or run, documents may not save correctly, etc.

Today, viruses are somewhat less common than network-borne worms, due to the popularity of the Internet. Anti-virus software, originally designed to protect computers from viruses, has in turn expanded to cover worms and other threats such as spyware, identity theft and adware.

Email viruses work in the same way as a regular virus however, an e-mail virus will use an e-mail message as a mode of transport, and usually will copy itself by automatically mailing itself to hundreds of people in the victim's address book.  Most email viruses can only infect files once opened.  Usually the virus will originate from an email attachment, making it ever important to only open attachments from trusted sources. 

A computer virus will pass from one computer to another like a real life biological virus passes from person to person. For example, it is estimated by experts that the Mydoom worm infected a quarter-million computers in a single day in January of 2004. In March of 1999, the Melissa virus spread so rapidly that it forced Microsoft and a number of other very large companies to completely turn off their e-mail systems until the virus could be dealt with. Another example is the ILOVEYOU virus which occurred in 2000 had a similarly disastrous effect.

 A Trojan horse is just a computer program. The program pretends to do one thing (like claim to be a picture) but actually its a malicious program that does damage when it starts.  Trojan horses cannot replicate automatically like viruses, but their impact can be much worse and they have proven to be much harder to remove from an infected system.  They basically have the same effect on the performance of your computer as a virus would, consuming resources and generally making your computer much less efficient.

 Worms copy themselves and infect files much like a virus however, a worm is a piece of software that uses computer networks and security flaws to create copies of itself. A copy of the worm will scan the network for any other machine that has a specific security flaw. It replicates itself to the new machine using the security flaw, and then begins scanning and replicating anew.  This can affect not only computers on the same local area network (LAN) but can also affect computers on the Wide Area Network (WAN) which is the network your ISP uses to provide you with internet service.  In essence a worm is a virus that copies itself onto other machines instead of just to other programs and areas on your computer. 

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