Waves are the result of energy being transferred from one place to another, such as from the wind to the surface of the sea. The most familiar of waves are the ones caused by wind on the ocean's surface. However there are several other types of waves that have different causes including tsunamis, the result of underwater earthquakes and internal waves, which are waves that travel underwater. Tides can also be included as a type of wave, caused by the gravitational pulls of the moon, the sun and the earth.
Groups of waves are made up of several crests that are separated by troughs. The time between each wave crest is known as the period. The period is used to classify waves into types. The smallest are ripples with a period of less than .5 seconds and the largest are tsunamis, whose period can be hours long. Between those two types are chop and swell, the most common type of surface waves. When wave groups meet each other they either add to or cancel each other out. Occasionally two or more wave will meld forming a giant or "rogue" wave.
Waves are the result wind energy being imparted to the ocean's surface through friction and pressure. As wind levels increase the surface of the ocean shifts from flat and smooth to rough, first in the form of ripples. If the wind continues to increase the ocean will soon be a choppy sea with waves that are 4-20in in height. How rough the sea gets depends on three wind factors: how fast the wind is blowing, the duration the wind blows for, and the area that the wind is blowing over.
In the area the wind blows over different types and heights of waves are generated and will begin to interact with each other. As waves move out of the area the wind is blowing in, known as the fetch, they become smaller and more regularly spaced. Wind-generated waves maintain a constant speed unaffected by the depth if they are crossing the open ocean. Only tsunamis are affected by the depth of the water.
When waves begin to reach the shore the motion that they generate deep under the ocean begins to interact with the ocean floor. Interacting with the sea floor causes the waves to slow down and the crests to bunch up together, called shoaling. Waves break in one of two ways, they either break on flat shores dissipating their energy gradually or they break on steep shores where the crest curls and falls over the front of the advancing wave causing the whole wave to collapse.