Facebook apps come in dozens of varieties with hundreds of examples in each category. There are quizzes, games, tools and other apps that let you rank everything from your favorite albums to the celebrities you'd like to meet. Each of these apps requires you to install a few lines of code to your Facebook profile. From your point of view, all you have to do is click a button on a page, indicate that you accept the user agreement and install the app.
That was a problem with some early Facebook applications. In order to function, most applications need to access some of your information. Early apps would often access far more information than they required. That meant the developers of those apps could access a great deal of personal information about users. Facebook tried to put a stop to this and demanded that developers only request access to information that was necessary for the app to work the way it should.