1) Applets 2) Apps 3) Usages 4) Uses 1) Computer programs 
Any complex software the user buys to accomplish a repetitive job that is unrelated to the basic nature of his own goals. A word-processing program is a good example, since it is mathematically complex, but its internal operations are functionally irrelevant to the user who is writing letters or a book. (WW)
Users can add applications to their profiles, pages, and groups based on interest or functionality. There are dozens upon dozens of applications to choose from. Some are built by Facebook. Most are built by external developers.