Above that is the stratosphere. Above that is the mesosphere which contains the ozone layer. Above that is the thermosphere. The upper thermosphere is known as the exosphere. Atmospheric gas is slowly leaking out into space through the exosphere region.
Ozone is a gas of O molecules.
These molecules are important for life because they are effective at absorbing harmful solar ultraviolet radiation. The Chlorine is especially effective in destroying O . Its origin and maintenance is thought to come from dynamo activity in the core. Interestingly, this means that whether a body does or does not have a magnetic field, and if it does how strong the field is, tells us something about the 'unseen' interiors of planets and moons. The Earth is extremely active.