A virus is a small program that is written specifically to cause destruction, disruption and data loss on your computer. The virus does this by using programming "back doors" that exploit weaknesses in your Operating System (eg. Windows XP) to automatically execute without your knowledge. They are usually noticed by their symptoms, which are, among others, slowing down of your computer, programs doing things they've never done before, files and directories and programs going missing, new programs suddenly appearing, and in the worst case scenario, computer hanging or crashing; and in the ultimate worst case scenario they can wipe your hard drive clean, so all your data is gone.
Worms and Trojans are also basically viruses, and they come into your computer usually via emails and email attachments, and document macros. (Doc macros are things that automate certain repetitive tasks in your documents. In fact, they are little miniature programs, and as such, they can be infected by a virus). These types of viruses can also cruise into your computer off the internet all by themselves. They do this by scanning computers for open ports, and then enter and take up residence. Worms tend to make themselves part of other programs, altering their operation. Trojans disguise themselves as something normal, while in fact being malicious.
In newer computers, Flash ROM is used and can be infected. Flash Rom is Read Only Memory that can be "Flashed," or updated, over the internet, by the manufacturer. However, virus programmers have also been able to "Flash" ROMs with undesirable results. So be very careful when flashing ROMs, and don't do it unless you have to.
It is important to get an anti-virus programme installed, and regularly updated, on your computer before you go onto the internet, to never open an unknown email, and to also get a firewall installed and configured. Also, stay away from dubious sites, always bearing in mind that "nothing is for free." Be wary of macros, of email attachments (even from friends) and free software. Free MP3 and music sites, and of course porn sites, are especially dangerous, teeming with viruses. Also avoid illegally copied software.
Keep your operating system updated with the latest security patches. Run anti-virus scans regularly. Back up your important data to DVDs and write the date on them. Then you should be pretty much covered.