Ozzfest is an annual summer music festival headlined by, as the name implies, Ozzy Osbourne (in the past few years the headline act has been Black Sabbath a few times instead, with Ozzy reuniting with his former bandmates) and organised by Ozzy and his wife/manager Sharon. Aside from Ozzy/Black Sabbath, the festival features primarily heavy metal and hard rock acts. Repeat performers include Primus, Slayer, Godsmack, et al. Often the festival ends up the highest grossing summer concert event in whichever year.
Ozzfest usually has a lot of bands playing with most of the better known groups appearing on the main or 'first' stage and some more up and coming-type acts on the 'second stage' (some of who end up playing the first stage in later years). Ozzfest begins its tours in Europe in June or July then finishes up in the United States. The final tour date is typically somewhere in southern California. The first tour was in 1996 with Ozzy (of course), Slayer, Sepultura, Biohazard, Fear Factory, Neurosis, Powerman 5000, Earth Crisis, Cellophane, and Coal Chamber and did not, as far as I can tell (I've been having some trouble finding much information on the first tour), have a second stage setup. All the tours in years following did. Davidian tells me that the first "tour" consisted of only a single show in Phoenix, Arizona.
When I went in 1999 the festival began around 11am (though I didn't get there until around 3 or 4pm) and ended twelve hours later. All the aggressive music and people and big lawn areas with mud and alcohol for sale and other substances sneaked in and whatever I've left out tends to make the festival very hectic and rambunctious but, usually, in a good way. In '99 at the San Bernadino show (the final show of the tour) the crowd ended up lifting the first three or four rows of seats down in the front off the ground. Security, rather than try to fix this, just made sure the rows of chairs (most of them were all attached to each other so they all had to come up together) were carefully lifted out of the way. (This is what you get for sticking chairs in the pit area, Blockbuster!)
I also managed to improve my seating arrangements when a bunch of us in the loge section (behind the pit, second best seating area) decided to rush the gate down into the pit area. Only a few of us got through but it was fun hurtling through the box seating section at the rear of the pit trying to evade security. The area the front rows of seating had been removed from was pretty rough but there wasn't much in the way of actual mosh pits there (most of that was back in the lawn section). It was great being about five feet from the stage for the last two bands (Rob Zombie and Black Sabbath). Despite the impromptu chair removal and gate-rushing, the security guards actually seemed to be handling everything pretty calmly (well, as calmly as can be expected) instead of the usual dicks many of them are at other concerts I've been to.
The festival brought together an interesting crowd. A lot of young and middle-aged rockers (Black Sabbath probably drew most of the older crowd) right along side each other. There's also a lot of subcultures mingling at Ozzfest (headbanger types, goths, people into nu-metal or rap metal or whatever it's called, et al) since Ozzfest features both older hard rock acts and newer bands alike. All in all one of the best concert experiences if you're into this type of music.
For information on the current year's line-up and tour dates, check ozzfest.com.