The digital signal is one which only consists of two states i.e. logic '1'   (+5 volts) and logic '0' (0 volts). Various electronic blocks use logic and these form the basis of a microcomputer system. Let us tackle theses simple blocks. The basis of maths is on logic operations namely AND, OR, NOT. Consider the electronic circuits below: 

If S1 and S2 are closed the current might flow to B, this is termed the AND function

 
If S1 or S2 are closed the current might flow to B, this is termed the OR function

 
 If S1 is pushed the current will not flow to B, this is termed the NOT function

 
In order to use these functions the idea of a truth table is used, where logic '1' is true and logic '0' is false. Note that these are also given electronic symbols and mathematical functions. 

By combining these gates we may derive an alternative set of logic gates.