From Microsoft's website...
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Pocket PC vs. Handheld PC
This is a question that often crops up: What is the difference between the Pocket PC and the Handheld PC? The answer is relatively simple: the Handheld PC comes with a keyboard and half- (320 x 480) or full-size (640 x 480) video graphics adaptor (VGA) screen, similar to a mini-notebook computer. The Pocket PC has a quarter-size VGA screen (240 x 320), and accepts input by stylus entry.

Windows CE vs. Pocket PC
This is probably one of the most common questions that appears in various discussion forums about the Pocket PC: What is the difference between Windows CE and the Pocket PC?
Well, the Pocket PC is a device that runs Windows CE 3.0. It includes numerous additional programs, such as Inbox, Notes, Money, and Tasks, as well as a new graphical user interface (GUI) better designed for the quarter-size VGA screen. Windows CE is an operating system that can be custom compiled using the Platform Builder to work on a number of platforms by developers, including custom machinery control systems.

The term “Windows for Pocket PC” refers to the software on the device, whereas the term “Windows Powered Pocket PC” refers to the actual device running the Windows for Pocket PC software.
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If I understand correctly a Pocket PC device is uniquely contructed to work with a set of standard tools and drivers built on top CE 3.0. Devices that are constructed differently won't run Pocket PC software.