Myopia is also known as near-sightedness or short-sightedness and simply means that the eye can see better close up than it can far away.  It is caused by the focal length of the eye's optics (the combination of cornea, aqueous, crystalline lens and vitreous) is shorter than the physical length of the eyeball.

It develops in susceptible individuals (those who are genetically predisposed to develop myopia, which may include most humans) in the early years when the eyes are subjected to long periods of near focusing. It is actually an adaptation to the near point which can be beneficial in small amounts as the person ages.

The lenses that correct myopia are concave, or thicker at the edges and thinner in the center, and are minifying (they make things look somewhat smaller).

It is possible that during the progression of myopia (which varies from early childhood into the late 20s) the full time wearing of glasses or soft contacts prescribed for far vision may speed up the progression of this condition.  This is a controversial subject, but there is enough evidence to warrant consideration of doing extensive near work with a different correction when myopia is increasing, one that reduces the amount of accommodative (focusing) demand on the eyes.