In general, distributed computing is any computing that involves multiple computers which are remote from each other, where each has a role in a computation problem or information processing.
In business enterprises, distributed computing generally means putting various steps in a business process at the most efficient place within a network of computers. In the typical transaction using the 3-tier model, "user interface processing" is done at the PC user's location, "business processing" is done in a remote computer, and "database access and processing" is done in another computer that provides centralized access for many business processes. Typically, this kind of distributed computing uses the client/server communications model.
More recently, distributed computing is also used to refer to any large collaboration in which many individual PC owners allow some of their computer's processing time to be put at the service of a larger problem. The best-known example is the SETI@home project in which individual computer owners can volunteer some of their multitasking processing cycles (while concurrently still using their computer) to the Search for Extraterrestrial Intelligence (SETI) project. This computing-intensive problem uses your computer (and thousands of others) to download and search radio telescope data. One of the first uses of distributed computing was the breaking of a cryptographic code by a group that is now known as distributed.net.