Ways to avoid: Turn on automatic updates in Windows and in your browser of choice. This type of vulnerability is usually quickly patched by the browser or OS vendor, and so attackers have a very short window in which to use it against fully updated systems. You're probably not the target that they're going to use this rare and valuable zero-day against.
Web browsers are the primary target for many attackers these days, because so much sensitive data passes through them. From casual shopping to enterprise management systems to military operations, browsers have become the primary vehicle people use to access network-connected systems. Unfortunately, browsers have a long and storied history of vulnerabilities that have provided attackers with a lucrative and near-endless supply of victims upon which to prey. Quarri Technologies, Inc., a Web information security software company, has identified some of the top vulnerabilities attackers use against browsers.
Note: This slideshow is focused on browser vulnerabilities, not website vulnerabilities (SQL injection attacks, XSS, XSRF, et al). The distinction is subtle but important.