What is non-functional testing?

Nonfunctional testing deals with testing parameters other than features of an application, these include performance, reliability, security, integrity, scalability, portability, etc.
Example: In case of desktop software say calculator, how quickly it displays an output, for multiplication of four digit numerals. In web based application, how quickly an image is rendered on the webpage, or if site is available 24×7 or is subjected to frequent maintenance, making it non-available to end users.