The Internet's version of the phone book is called DNS (Domain Name Service). It converts your typed in address of "google.com" to the real address of "64.233.167.99" and then tells your computer to go to that address.

Your ISP (Internet Service Provider) keeps a trusted copy of that "phone book."

Many computers have become infected simply by browsing less-reputable web pages.

This is called a "drive-by" download. Improperly protected computers and browsers can be opened up to advertising pop-up windows, have keyloggers installed (recording all keystrokes) or be controlled remotely by someone else to attack other computers, or to send spam email.

Setting appropriate security settings on your browser can limit or deny access to such programs, but it can affect how sites operate.
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