There is a lot of great information about the ozone/UV interaction within the Earth's atmosphere on the Wikipedia "ozone" page:

http://en.wikipedia.org/wiki/Ozone#Ozone_in_Earth.27s_atmosphere

As described there:

..................................quote..................................

Ozone in the stratosphere is mostly produced from ultraviolet rays reacting with oxygen:

O2 + photon(radiation< 240 nm) → 2 O {followed by}
O + O2 → O3

It is destroyed by the reaction with atomic oxygen:

O3 + O → 2 O2

................................../quote..................................

So, while the ozone layer is indeed the region of the atmosphere where ultraviolet photons are filtered, ozone molecules themselves aren't involved in the actual filtering process (but are instead created as a byproduct of it). UV photons strike and break apart diatomic oxygen, creating two free oxygen atoms. One of these free atoms interacts with an O2 molecule, thereby creating ozone, i.e. O3:

http://upload.wikimedia.org/wikipedia/commons/7/7a/Ozone-3D-vdW.png

As an aside, since the region of the atmosphere containing larger than average quantities of ozone (a.k.a., the ozone layer) is so efficient at filtering UV light, astronomers who want to observe very hot objects (which also means they are UV-bright) such as massive main-sequence and neutron stars have to perform their observations from space-based telescopes (i.e. above the absorbing atmosphere).
Source(s):
http://en.wikipedia.org/wiki/Ozone

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