Facebook applications are small programs that work inside Facebook. They're similar to Web browser plug-ins (like video players) in that they let you do a little something extra—something you couldn't do before you installed them. They're easy to install and appear on your Facebook Applications menu.
If you've had a chance to put Facebook through its paces, you're probably already familiar with the site's built-in applications, which include Groups, Events, Photos, and Marketplace. But people who aren't on the Facebook design team—folks known as "third-party developers"—have written about a bazillion others, such as iLike (which lets you add music clips to your profile) and Weekly Schedule (which lets you coordinate a graphic version of your schedule with your co-workers' or friends').
Nearly all the initial third-party Facebook applications are humorous time-wasters, like the ones that let you spray-paint graffiti on someone's wall or hurl virtual sheep at your friends in lieu of a sedate poke (Poking). But you can expect to see more serious-minded, business-friendly applications now that Facebook's enormous (and growing) membership is attracting high-dollar advertisers. Professional Profile, for example, lets you post and edit your resumé on Facebook, then track who views it. Chat-Instant Messaging lets you add an instant-messaging box to your profile so you can communicate with co-workers without ever leaving Facebook.
Facebook applications are ...