UV printing is a relativly new technique of printing which works more or less like your home printer only using different inks and UV light to cure the prints. The same UV light is used at dentist chair to cure your fillings. There's no limi in complexity of the design, number of colours. Every object that fits under the printing head and has a flat surface can be marked with this technique.
A key factor of UV printing is distance between printers head and the object, the smaller the better, therefore the limitations to flat surface mentioned above. UV printing is often used for printig notebooks, powerbanks, frisbees and USB's