When someone asks me, "What would happen if you were exposed to the vacuum of space without a suit?", I give the sweet, simple, short answer: you die. With enough time, I dig into the medium-length answer: you die, horribly. Given the opportunity to write a few paragraphs, I relish in the long answer. That long answer is pretty interesting, in a horrible kind of way.
The first thing to happen is the violent evaporation of any liquids (sweat, tears, mucus, oils, etc.) on the surface of your skin. Gross, but survivable.
The second thing to happen is a general puffiness as dissolved gases in your bloodstream collect and expand. Your skin is pretty good at keeping your insides inside, so you don't pop, but you do swell to around twice your usual size. Incredibly gross, but again, survivable.
If you're unlucky enough to be exposed to raw sunlight, those unfiltered UVs will do a number on your skin. Supremely gross, but nothing a gallon of aloe can't fix.
What gets you is the oxygen, or rather lack thereof. There's no air to breathe, but your heart doesn't get the memo. It keep pumping blood around your body. The hemoglobin train keeps chugging along, but doesn't pick up any oxygen at Lung Central Station. In about 10-15 seconds your brain realizes it's not getting any fresh supply and goes into safe mode (i.e., unconsciousness).
If you're pulled back in time, you can still be revived. But left for another minute or two: major organ failure. You know, because of no oxygen, which is kind of important.
And one last public safety tip: do *not* hold your breath. The meat valves in your throat were not meant to hold a lungful of air against a vacuum, and if you fight it your delicate precious lungs will rupture. Just let it go.
