This smart, wealthy boy named Adrian Veidt idolized Alexander the Great and Ramesses II for their intellect and power. He donned a costume at age 19 and became a vigilante, eventually catching the eye of the Crimebusters (a sort of interim group between the Minutemen and Watchmen that never really got off the ground).
But when public opinion turned against vigilantes, he retired and became a great humanitarian (at least publicly). In secret, he planned to unite the world via a catastrophic event that would kill millions. When The Comedian discovered this plan, Ozymandias killed him, which kicked off the events of the Watchmen film.
An older Ozymandias, played by Jeremy Irons, will appear on the HBO Watchmen series.