The Central Processing Unit, or CPU directs the activities of all other hardware. The CPU is in turn directed by programs, which are sequences of instructions. Put simply, a CPU converts programs into actions.
Modern computers usually have more than one CPU, so they are capable of running multiple processes at the same time. The term CPU has become somewhat ambiguous, because a single CPU chip often contains multiple independent CPU circuits. Hence, the term CPU sometimes means the whole CPU chip, and sometimes means one of the CPU circuits within the chip. To avoid confusion, most people now use the term core to refer to one of multiple CPU circuits within a chip.