Waves are primarily caused by the wind and its action on the surface of the water. Their height depends on how long the wind has been blowing and also on the strength of the wind. Waves formed by the wind blowing locally are termed "sea". Waves formed by the wind blowing at a distance from the place of observation are termed "swell".
Some waves result from earthquakes or underwater seaquakes and on approaching shallow water they become abnormally high and begin to break with great violence causing enormous devastation and loss of life. They are termed "tsunami" and we will all remember the tragic waves caused my a seaquake near Sumatra on Dec 26th, 2004, which claimed the lives of nearly 300,000 people in South-East Asia.


The following terms are frequently used in connection with waves:

- the length of a wave, that is the horizontal distance from crest to frest or trough to trough. If the distances between the crests of waves are far apart, the sea is termed "a long sea". When the crests are close together the sea is termed "a short sea", like for example in the Baltic Sea.
- the height of wave, that is the vertical distance from trough to crest.
- the period of a wave, that is the time between the passages of two successive wave crests or troughs past a fixed point.
- the velocity of a wave, that is the rate at which the crest travels.