You will control all of the members of your marine squadron simultaneously, with the ability to alternate from one to another whenever you want. Each member has their own individual special ability, which means that you can use one to launch grenades, while another, for example, never has to recharge.
In addition to controlling your marines in first person, you can view a tactical map of the zone where you're located, so that you can give rapid orders to your men. That helps your team move quickly through the ship, without you having to control each member directly.
Your men will always shoot when confronted by an enemy, but you have to be very careful about these situations, since a sole marine may not be able to survive an encounter with a dangerous creature.
Derelict is a realtime strategy action game that, despite not having great graphics (both the graphics and sound are lacking), offers a unique and quite spectacular playing experience. It's something between horror and science fiction, and that's simply awesome.