Afunction of the form f(x) = mx + b, where m and b are real numbers, is called a function.

This is a linear function.  Note that the independent variable, x, has the first power.  Were the function g(x) = mx^2 + b, you'd have a quadratic function.

It's important that you be able to envision functions such as the constant function, linear functions, quadratic functions and cubing functions quickly when you encounter them.