Waves are a 'disturbance' that travel in space carrying energy with them. We have some idea of what a wave is, now let us try to refine it. First
let us see a classification and a few examples of waves.
Waves which require a medium for their propagation are called mechanical waves.
Waves on a string
Electromagnetic waves do not require a medium for their propagation. They propagate through oscillations of electric and magnetic fields. Light, X-rays, Ultra Violet rays, Infra-red rays are all examples of Electromagnetic waves. However, while the examples in the case of mechanical waves referred to different kinds of waves, here these waves differ only in terms of their frequency of oscillation, and this gives them different properties.
The de Broglie hypothesis (1925) says that every particle has a wave nature. Such waves show their effect only at very microscopic levels and are not observable in daily-life situations. This idea of wave nature of matter is very significant in quantum physics.