Fusebox applications are modeled around an electrical Fusebox in a House. For example, the electrical circuit for your kitchen is separated from your bathrooms and also separated from your living room. In the end, your house has electricity, to the common family they never notices that the electricity is really separated. When little joey decides it would be funny to put a fork in the electrical socket in the kitchen and he blows out the fuse. So the power goes out in the kitchen but it doesn't go out in the living room. This is similar to a software program, when a user does something that the developer wasn't expecting them to do the program throws an error. With Fusebox we separate each application from the other application's in the home thus allowing circuits to be blown without taking out ALL the power in the home application.
There are two types of applications in a large scale Fusebox application.
Circuit Application - If you've ever written a Fusebox application before you've probably written a Circuit application. This generally consists of a single directory of files (although it doesn't have to be limited to this). A circuit application generally does a few related tasks.
Home Application - This is the overall application, it is made up many circuit applications that are tied together with an interface. The common user does not realize they are using a circuit applications they are under the impression that they are using the Home application. The entire site is made up of many many circuit applications that do various specific tasks such as searching, information, product details, purchasing etc. Although to the common user whether they are searching, or checking they are still in the home. This is similar to a real life home. Whether a person plugs a television into an outlet in the kitchen or in the living room, they still get power, because they are in their home. (assuming they paid the power bill :). It is important to realize that circuit applications may contain bugs in them.... but if they do, it is possible to remove them from the Home application without bringing the rest of the Home application down. It is also possible to go the other direction. When a new Circuit is added to the Home application it can be added without affecting the other circuit applications.