What do these abbreviations in the name of Arduinos (or maybe processors?) mean? I remember '16AU' and 'P-20', but there are more out there...
Packages for AVR devices - see the Atmel data sheets,
PU - DIP package AU - TQFP package 20 - 20MHz 16 - 16MHz P - low power version ?
There will be more, as Linuxuser says it's in the data sheets although sometimes hard to find.
Easy to find - Section 32 of the ATmega48PA/88PA/168PA/328P datasheet, page 430.
I believe P = plastic package