Operating systems such as Microsoft Windows and Apple Mac OS X come with large complements of application programs. Nonetheless, most people will install other applications besides those that came with the operating system: web browsers, word processors, spreadsheets, music managers, media players, games, ... the list is endless. Although the makers of these applications try to produce quality products, it's a fact of life that, from time to time, defects, or "bugs," will be discovered. Although some application software includes functionality that checks for bug fixes (usually called security patches) and installs them automatically, most do not. Instead, the application creators release new versions of the software, which you have to download and install to obtain the fixes. Keeping your applications up-to-date is just as important as updating your operating system; if you don't, it's much more likely that your computer could be infected with malicious software (viruses, worms, etc.) or accessed by a hacker. This is especially true of web browsers and streaming media players such as Flash and Quicktime.
See more information in our Security Handbook to learn how to keep your software up-to-date.