A block is a piece of middleware that is contained in a single JavaScript file (.js), which provides methods for user generated code to invoke. It also has an associated XML manifest file which describes more about the block. A block may also make use of resource files such as XAML files, images, etc.
A block can act as a middleman between externally provisioned services such as web services, or it may simply be a library of useful functions, e.g. a function that calculates the area of a circle given a radius. A block can also act as a display surface: something which takes data from other blocks and displays it in a meaningful manner, and allows the user to interact with it. In this guide we will use Soapbox (
http://soapbox.msn.com/) as the example web service for which we are building a block.