Registers are storage locations internal the the processor.
CPU instructions operate on these values directly. On RISC processors, all data must be moved into a register before it can be operated. Also, there are registers that are set aside for certain tasks, these generally include a program counter, stack, and flags. Each register also has a size that determines the maximum amount of data that can be processed at a time. The registers on Pentium chips, for example, are 32 bits.