A function table displays the relationship between the inputs and outputs of a specified function. A function table will also follow the rules of a function in that every input only produces one output.
Inputs are more popularly called the domain of a function. It is used very often in math to restrict the domain to only the real numbers or to only the integers.
Range or Image
Outputs are more popularly called the range or image of a function. While the domain can be easily restricted, it is much harder to do so with the image because it has more flexibility.
An example of a function is one where it takes a number and doubles it. For example, one input is 7 while the corresponding output is 14. Many more complex functions exist, even involving imaginary numbers.