The event itself was an Eagles of Death Metal rock concert. That is morbidly ironic...even the band has said they're scattered still trying to gather the group up after the attack.
According to what I've read and watched on the news, the assailants also wore suicide jackets that would be set to blow at the wearer's command. Seven went off, one got shot down by authorities as he escaped, and as soon as he hit the ground dead, it too exploded. These people weren't going to leave a testimony willingly, and intended to die if all else failed.
This has also congressed a large number of "concealed firearms" discussions locally where I am, and that in itself would be a terrible idea, given it's a large event. Not only would you have crowds of crowds of people who could easily slip it out and fire a round with your gun, but look at the 8th terrorist and how that ended. You shoot, you kill more than the criminal.
Quite truthfully, we won't know much more until later. Things are turning to ♥♥♥♥ real fast.
EDIT: I was wrong on the bomb situation.
The four who attacked the Bataclan theatre wore black clothing and used AK-47 assault rifles. Three of those attackers killed themselves with their explosive belts during the police raid on the theatre, while the fourth was killed by police gunfire just before his belt detonated. Three other attackers detonated their belts in the suicide bombings near the Stade de France, while the eight attacker detonated his belt on the Boulevard Voltaire near the Bataclan theatre.
Without a doubt, this was organized. The question is, who?