When two waves are traveling in the same medium but in opposite directions, different interference patterns form. If the two waves moving in opposite directions have the same wavelength, an interference pattern known as a standing wave can form.
In the version of the simulation shown above, the red and blue waves are moving in the same medium, but in opposite directions. When the two waves have different wavelengths their sum is a complex traveling wave.
Now set both of the wavelengths to the same value (this can also be done by clicking the "Show Standing Wave" button). Their sum is a special type of wave interference known as a standing wave. It is called a standing wave because it doesn't move in the direction of either wave.
Some points on the standing wave do not move at all, those points are called Nodes. The points on the standing wave that undergo the greatest displacements are called antinodes. Use the checkbox to observe the nodes and antinodes.