Processing devices in a computer are responsible for controlling the storage and retrieval of information. The information is processed by the computer processor (CPU), which performs data calculations, data comparisons, and data copying with the information from the processing devices. The CPU then saves that information to the computer memory (RAM).
Today's computers contain multiple processing devices. Any device that deals with the input and output of information does some form of processing on that information, some with more complexity than others. The one processing device that most people think of is the CPU, but devices like the graphics card, computer memory, network card, and others are processing devices as well.