The Internet works by sending information from computer to computer until the information reaches its destination. When information is sent from point A to point B, every computer in between has an opportunity to look at what's being sent. This can pose a security problem.

For example, suppose you are viewing a clothing catalog on the World Wide Web and you decide to buy a shirt. This requires that you type information into an order form, including your credit-card number. You know the clothing company in question is reputable, so you type your credit-card number and other information, and then send the completed form. Your information passes from computer to computer on its way to the clothing company. But suppose one of the computers in between has been infiltrated by criminals who watch the data passing through that computer until they see something interesting, such as your credit-card number.

Another security problem relates to how files and programs are sent and received between Web sites and your computer. With no security protection, you could run or download files and programs from the Internet that can damage your computer and the information stored on it.

How often do things like this happen? It's hard to say, but the important thing is that they're technically possible. And, as the Internet grows, they could happen more and more.