What are the differents alternatives for triggering all thoses off camera flashes at the same time ?

Usually you want to trigger them by your master flash on-camera. But for completeness, here's the all your options:

1.
Wiring the flashes. Use the sync port of your cam to trigger a hub, which then triggers x flashes via cable. Your flash needs to a pc jack for the trigger cable.
Advantage: flashes are not irritated by light noise or radio noise, so in a big pop concert that might be the way to do it. Otherwise: discard this option.

2.
Optical slaves: All speedlights work as optical slaves. Your camera flash acts as the master and triggers, the other flashes see this and follow. Canon and Nikon flashes even do that with the support of ETTL / ITTL. This is the way you want to do it in 95% of all shoots.
Advantages: Native TTL is supported, no cabling or other funky third party things needed, easy to use

3.
Radio triggers: If you are shooting in bright daylight, then the optical slaves cannot pick up your master. In this cases you attache a radio trigger master to your camera and attache radios as slaves to the off camera flashes. However, TTL is usually not supported, at least not by those triggering systems that I found usable.
Advantage: Triggering is working rocksolid, even in bright daylight
Check out this video to see how I am using my favorite radio remote triggers: the phottix atlas. 

 Thanks to Martin Clermont for the question.