The arithmetic and logic unit is a unit of central processing unit where all arithmetic and logical operations are carried out.The basic arithmetic operations includes;addition,subtraction,multiplication and division.Logic operations are based on the computers capacity to compare two or more values. For example,it may compare whether a piece of data is greater than or less than, equal to  or not equal to etc.
In order for the ALU to be able to process data,it has special temporary storage locations called registers,which holds the data just before processing.It also holds the results after processing.