PDAs are essentially very small computers, and as such they require the same type of hardware as a computer: a processor, memory, and interfaces to connect to the outside world. All this has to be designed to consume as little power as possible so that the device's battery doesn't run out.
The processor is far less important in a mobile device than in a PC. While desktop PCs are frequently marketed with a large number of MHz (or, increasingly, GHz) as their main selling point, it can be very difficult to discover the speed and even the make of the processor inside a PDA or phone.
The reason is that there is no single common hardware or software platform, so it is harder to make comparisons. The Palm and the PocketPC devices ...