The ozone layer, part of what is known as the stratosphere layer of our atmosphere, is mostly identical to the air you breathe, if a little dryer (less water vapor). That is to say it's about 78% nitrogen, 21% oxygen, with the rest being small amounts of various other gases such as argon and carbon dioxide.

What makes the ozone layer special is that a very small amount of its oxygen (every one in several million atoms) is in a special form.

"Like many of its neighbor elements on the periodic table , oxygen atoms want to pair up and are usually found in the "O₂" form. It takes high energy light from the sun (called ultraviolet) to break this bond between oxygen pairs, but once an O₂ molecule is split into two separate Os, it's a lot easier for each O to join another O₂ and form a triplet than to find another rare individual O. These triplets, O₃ , are called ozone and being unstable usually decay into other forms, such as two triplets forming three pairs.

This process is cyclical with ozone constantly being created and destroyed, thankfully absorbing the vast majority of harmful ultraviolet light in the process, which is why our ozone layer is so important. As bonds are broken, the energy from the sun is turned into heat, heating up the stratosphere.