Stream is the base class for character and binary based streams. It is not called directly, but invoked whenever you use a function that relies on it. The Particle Stream Class is based on the Arduino Stream Class.
Stream defines the reading functions in Particle. When using any core functionality that uses a read() or similar method, you can safely assume it calls on the Stream class. For functions like print(), Stream inherits from the Print class.
Some of the Particle classes that rely on Stream include :