What Locke1689 said. If you're not familiar with big O notation, linear is like f(x) = 2x, quadratic is like f(x) = x^2, and exponential is like f(x) = 2^x. There's a huge difference between quadratic and exponential: exponential grows significantly faster as x increases. For computing, the difference is significant. Cobham's thesis says that polynomial algorithms (which includes quadratic) are reasonable to perform, while exponential algorithms aren't.