Introduction of Logic Gates and True Tables

Logic Gates 

A logic gate is physical device implementing a Boolean function that perform a logical operation on one or more logical inputs, and produces a single logical output.  Logic gates are primarily implemented by using diodes or transistors acting as electronic switches. With amplification, logic gates can be cascaded in same way that Boolean functions can be composed and allowing the construction of a physical model of all Boolean logic. Therefore, all of the algorithms and mathematics can be described with Boolean logic.

Truth Tables

Truth tables help understand the behavior of logic gates.
They show how the inputs of a logic gate relate to its outputs.
The gate inputs are shown in the left columns of the table with all possible inputs combination. This is done by making the inputs count up in binary.
The gate outputs are shown in right hand side column.

Example:
Consider a logic function with three inputs, A, B, and C, and three outputs D, E, and F. The function is defined as follows: D is true if at least one input is true, E is true if exactly two inputs are true, and F is true only if all three inputs are true. 

Truth table with n(3), contain 2³ = 8 entries

N= 2n             N = number of possible input combination

n = the number of input variables

2 inputs -> 22 = 4 combinational

3 inputs -> 23 = 8 combinational

4 inputs -> 24 = 16 combinational