Explain how a standing wave is formed

A standing wave is formed when two waves travelling in oposite directions interfere with each other.  The waves will be exactly one half wavelength out of phase, and this phase difference allows them to inferfere with each other.  

At points of distructive interference, nodes are created as the waves effectively cancel each other out.  There will also be antinodes created where there is constructive interference, as the waves "add" together.  

It's important to remember that despite the fact that the wave appears to be standing still, it is infact made up of two travelling waves interfering.