ozone layer

A region of the stratosphere, between 15 and 30 kilometres in altitude, containing a relatively high concentration of ozone; it absorbs most solar ultraviolet radiation. The "ozone layer" contains more than 90% of the earth's ozone. Ozone is a corrosive, light blue gas with a smell something like burning electrical wiring. The atmosphere at this altitude is still about 78% nitrogen, 21% oxygen, and the peak ozone concentration is about 9 ppm (or 0.0009%). Other things (water vapor, carbon dioxide, argon, and so on) are present there in small concentrations too.

Ozone itself is a triatomic molecule, composed of three oxygen atoms that bonded, unlike the oxygen we breathe, which are diatomic molecules, meaning two oxygen atoms.
The ozone layer is a region high in the atmosphere, containing this ozone, (concentrated in the lower stratosphere to a maximum of 9 ppm) that filters out most of the sun's dangerous ultraviolet rays (UV-B). UV-B is also absorbed by the DNA in all surface dwelling life on Earth, which causes cataracts, cancer, mutation, and reduces crop yields, and arable land.
An ozone hole also periodically forms, since ozone is unstable. The word "hole" is somewhat misleading. This is actually a diminished concentration of ozone due to the lack of sunlight and not a complete absence. An ozone hole forms over a pole, then later heals, once each year at the pole that is not receiving UV-C light from the Sun. The southern polar hole is larger than the northern polar hole, due to the polarity of Earth's magnetic field. Contaminants are suspected of making the hole larger, last longer, and contain less ozone, which is only an indication of the general "health" of the ozone layer. The hole itself forms in areas that are receiving no UV-B from the Sun either, so there are no lifeforms at risk from our Sun "beneath the hole"... every surface organism is at risk from a thinned ozone layer, but only when the Sun is above them.
The ozone layer is a region of the atmosphere where enough oxygen (and nitrogen too for some ozone production) is present to interact with very short wave UV (UV-C), and recombine to form some ozone. The ozone then blocks longer wave UV (UV-B). This layer is located roughly between 10 and 4 miles above the surface of the earth (depending on whether above the equator or above the poles), with highest ozone concentrations in a region that is alternatively called the "lower stratosphere", the "tropopause", or the "ozone layer".
If you took all the ozone in an entire column from the ground to infinity, and compressed it to STP (standard temperature and pressure, or 0°C at 1 atmosphere pressure) it would be a layer about 3 mm thick. Less than 1/3 as much in the Antarctic ozone hole when it is winter there.