In mathematics, a function's domain is all the possible inputs that the function can accept without breaking and the range is all the possible outputs. A real life example of this is using a simple calculator to add two numbers together. The function is the sum of "n" plus "m," the domain is all real numbers and the output is also all real numbers.
Another possible example is using a calculator to divide two numbers: the function is the quotient of "n" divided by "m," the range is all real numbers such that "m" does not equal zero, and the output is all real numbers. The reason that the domain has the restriction that "m" cannot equal zero is because dividing by zero is undefined and would break the function.
Beginning math students often confuse domain and range. A simple mnemonic device for remembering this is thinking about the domain name of a website. When someone wants to visit a website, they input the domain name into the address bar of the web browser. By thinking about this it is easy to remember that the domain is the input, meaning the range has to be the output. Alternatively, the student can think about a shooting range, where guns output bullets.