The ozone layer is that part of the stratosphere which contains ozone. Ozone is notable for its ability to absorb certain frequencies of ultraviolet radiation. The ozone layer is not very dense; if it were compressed to the density of the troposphere, it would be only a few millimeters thick.
Ozone in the earth's atmosphere is generally created by ultraviolet light striking oxygen molecules containing two oxygen atoms (O2), splitting them into individual oxygen atoms (atomic oxygen); the atomic oxygen then combines with unbroken O2 to create ozone, O3. The ozone molecule is also unstable and when ultraviolet light hits ozone it splits into a molecule of O2 and an atom of atomic oxygen, a continuing process called the ozone-oxygen cycle, thus creating an ozone layer in the stratosphere.
The ozone layer can be destroyed by the presence of atomic chlorine, fluorine or bromine in the atmosphere leading to the so-called ozone hole in the polar stratosphere during winter months; these elements are found in certain stable compounds, especially chlorofluorocarbons (CFCs) which may find their way to the stratosphere and there be liberated by the action of ultraviolet light on them. All the gases mentioned are denser than air, so they eventually diffuse to ground level and there are absorbed by reacting with almost anything organic, but have plenty of time to catalyze the breakdown of ozone in the meantime. Chlorine in particular is capable of breaking down approximately one hundred thousand times its molarity of ozone.
The concentration of atmospheric ozone in the ozone layer varies by a large factor worldwide, being thicker near the equator and thinner at the poles. Ozone levels, over the northern hemisphere, are dropping by ~4% per year. Approximately ~4.6% of the Earth's surface is not covered by the ozone layer; these are the ozone holes.
On August 2, 2003, scientists announced that the depletion of the ozone layer may be slowing down due to an international ban on chlorofluorocarbons.  Three satellites and three ground stations confirmed that the upper atmosphere ozone depletion rate has slowed down significantly during the past decade. The study was organized by the American Geophysical Union.