A processor register (CPU register) is one of a small set of data holding places that are part of the computer processor.
A register may hold an instruction, a storage address, or any kind of data (such as a bit sequence or individual characters). Some instructions specify registers as part of the instruction. For example, an instruction may specify that the contents of two defined registers be added together and then placed in a specified register.
A register must be large enough to hold an instruction - for example, in a 64-bit computer, a register must be 64 bits in length. In some computer designs, there are smaller registers - for example, half-registers - for shorter instructions. Depending on the processor design and language rules, registers may be numbered or have arbitrary names.
A processor typically contains multiple index registers, also known as address registers or registers of modification. The effective address of any entity in a computer includes the base, index, and relative addresses, all of which are stored in the index register. A shift register is another type. Bits enter the shift register at one end and emerge from the other end. Flip flops, also known as bistable gates, store and process the data.
See an introductory tutorial on processor registers: