Logical architecture.

Host provides root hub to which multiple devices may be attached.

Some of these devices may be other hubs providing a 
  Hierarchical tree structure.

A host may have more than one root hub providing a separate tree for 
 additional devices.

A device is assigned an id when attached. A root hub supports 127 devices.

A device can have up to 32 endpoints. Endpoints are designed into the device.

Endpoints are grouped together to form a function and a device can have
  multiple functions.  

  Web cam with mic. Video can be one function which uses 3 endpoints for color.
  And a separate function with one endpoint for sound.

  This is know as a composite device.

  A physical device can also treat different functions as separate virtual
    devices.

  Printer/Fax/Modem has an internal hub and may be given 3 ids. 
  This is a compound device.

Communication is performed via pipes. 

A pipe is a logical connection between host and an endpoint on a device.
  Most pipes are 'stream' and unidirectional.

  A pipe exists while information is being transmitted between device and
    host hub.

  Pipe 0 is a bi-directional 'message' pipe and is used for control and status.
    Each "device" will have a pipe 0.