An Arithmetic Logic Unit (ALU) is a fundamental building block of a central processing unit (CPU). The ALU is responsible for carrying out the computations involved in executing instructions from the CPU. It performs basic arithmetic, boolean logical operations and comparisons. The ALU is built up of low-level logic gates that can receive binary inputs in the form of 1s and 0s and carry out Boolean operations such as AND, OR, and NOT, as well as arithmetic operations like addition, subtraction, and multiplication. The results of the calculations are then stored in the CPU's internal registers, which are then sent back to the user, or sent to the output.