Most commonly they are attached to an email, or in a downloadable file that is available on the Internet. These viruses usually try to erase system files on a computer and render it useless. One of the latest viruses to be in the news was the Melissa Virus. This virus was developed to run on the end users' computer, but it was designed to crash the email servers by flooding them with more emails then they could handle. Software companies are fighting a never ending battle to stop the free distribution of their software so the website they are posted to are usually shut down pretty fast.
Hackers definitely do form an online community.
Everyone, from the largest computer companies, down to the novice computer user knows their presence. All computer users generally fear hackers even though most of them are not out to do any harm. But the ones that do end up being malicious, usually do enough damage do strike fear into everybody's mind. There are a few simple rules one can follow to protect yourself from some of the hacker's devious plot. The biggest thing the end user can do is to always have a virus program running and to keep it up to date.