You can get all this from the site, but the gist is as follows (from a skeptic/square's perspective)
Burning Man got its start in SF a few years back. It was an informal gathering of neo hippies and artists and others of "dressed in black" contingent. It moved to the Nevada desert after "outgrowing" its original site in Baker Beach.
It purports to be very "artistic" "free-spirited" gathering. The highlight is when the anarchist/artists build a giant effigy and set it aflame. It emphasizes "anarchy" and "freedom" and "art" and all the other things one would expect.
God, I am getting old to be so cynical.
Would El Guapo sponsor this event? Well the Republican equivalent is held several times a year at the local county fair grounds nationwide. They are called "Gun Shows"