Suppose you are enjoying spaceflight on your way to some distant planet when your spaceship is hit by a space rock, the hull is breached and you are sucked out into space without a suit. Bad situation.  What exactly happens to your body?

1. Your body has internal pressure that is offset by the pressure of the Earth’s atmosphere (or of your pressurized spaceship/spacesuit). Without that offsetting pressure you’d experience extreme and unpleasant internal swelling and ebullism which is the formation of bubbles of gas in your body fluids due to a lack of environmental pressure. You’d swell to approximately 2x your normal size. You won’t balloon to the point of exploding, though, since human skin and connective tissues are strong enough to keep you from bursting.
2. You will be without oxygen, obviously, and you’d lose consciousness in about 15 seconds. But what if you held your breath- couldn’t you retain consciousness for a few minutes like going underwater?  Holding your breath is a bad idea as the lack of external pressure would cause the gas inside your lungs to expand, which would rupture your lungs likely causing quick painful death. Thus, the first thing to do if you ever find yourself suddenly expelled into the vacuum of space is exhale.
3. You’d be horribly sunburned very quickly without Earth’s protective ozone layer.
4. Space is very cold – negative 454.81 degrees Fahrenheit. Surprisingly, being in this cold environment wouldn’t freeze you right away. Unlike being in water or air, there is no convection or conduction of heat. Thus, you’d lose heat by radiation – humans radiate heat at about the equivalent to a 100 watt bulb. So, you’d be super cold but you wouldn’t be completely frozen right away.
5. The saliva in your mouth and any other surface fluid would begin to boil. This is because water/fluids boil at lower temperatures at lower air pressures. In pressure-less space, the boiling point of fluids is below body temperature. This theory was inadvertently confirmed in 1965 when an astronaut was exposed to near vacuum in a test chamber and his suit sprung a leak. He remained conscious for about 14 seconds; his last sensation was bubbling on his tongue as his saliva boiled.  He survived the ordeal as the vacuum chamber was quickly re-pressurized.

All-in-all, being in space without a spacesuit or spaceship would be a pretty unpleasant experience. Assuming you didn’t hold your breath, you’d probably survive if you were rescued within about 2 minutes.