The 3D Printing pens are essentially a new category of devices that has appeared due to the big interest in consumer-oriented low cost 3D printers in the last few years. These devices are essentially a very simplified version of a 3D printer in a hand-held format or a more complex version of a glue gun. With a 3D printing pen you essentially get an extruder that you hold in your hand and move around to print stuff similar to how the extruder is being moved in a 3D printer by the precise mechanism. Obviously with a hand-held extruder and hand movements you cannot achieve any precision that is even remotely close to what a 3D printer can achieve, but that is not the goal here anyway. You can view 3D printing pens as more of an art oriented devices where it is not about achieving a high level of detail or much of a precision, but being able to let your imagination free and literally start drawing three-dimensional physical objects.