bit      The most fundamental unit of a computer.

 byte     It is fundamental addressable unit of RAM, and consists of 8 bits.

 nibble   Nibble is a hexadecimal value that contains 4 bits of information.

 word     It is a fixed amount of data processed by hardware of a processing unit.

 register Memory unit of a CPU

 alu      This unit in cpu do calculations and changes the data to create new 
          as directed by control unit.