Air pressure is what makes things go boom in space. Unprotected things that is. That's because air will try to stop at nothing to move to a place with lower air pressure.
Take a balloon, for example. Put in in your room. Nothing happens. Put it in space, and the high pressure air inside it wants to move to the zero air pressure of the vacuum of space. Since the balloons rubber skin is fragile, it pops. But space craft are reinforced and are meant to withstand the the huge air pressure trying to get out.
This pretty much my general knowledge of it. Sorry if the explanation is unclear :S