It depends on the country and the game. Big game driven hunts and bird hunts are normally a bit different. Also driven grouse or pheasant in the UK is different from red leged partridge in Spain, etc.

Contrary to a lot of commentary in the US about the "stiff formality of driven shoots" not all are this way. I've been on some driven boar and roe deer hunts in France that were not any more formal than the deer drives in Virginia or Pennsylvania. Likewise the one time I went driven partridge shooting it was pretty casual. I actually "stuck out" being one of the only shooters with a tie

Some general things: like Nitro well said, in general most places require you to put your gun back into a soft case between drives or moving between pegs or locations. In many countries the guns have to be cased in order to travel in vehicles etc. Most drives will have some sort of a signal to begin, its only at this point that you can load your weapon. Its normally a horn with one long blast, but this can vary. At the end of that particular drive there is another signal for the end where you immediately stop shooting and unload. In the one I was on it was two long blasts on the horn. Depending on place, location, game, there will be rules for zones of fire, where you can not fire, what you can shoot, etc Any good shoot will go through this with you at the begining.

For information specific on bird shooting in the British tradition, Robert Churchill's "Game Shooting" and Pete Blakeley's "Successful Shotgunning" have some sections on etiquette and things. Also Mike McIntosh has a short section in "More Shotguns and Shooting" on driven shooting.

Hope this helps! Its great fun!