Contrary to popular belief, in the short term nothing very much would happen. NASA has actually tested this, and Arthur C Clarke has incorporated the results into a couple of his stories. If the astronaut did not open his/her mouth wide so that the air in the lungs could escape, there would be a serious chance of damage to the lungs and chest. It would be necessary to open wide and let the air rush out. Obviously, with no air around, the astronaut could not breathe, so he/she would die within a small number of minutes - a number that could be extended by pre-loading with oxygen. If the bare skin were exposed to the sun, a significant amount of sunburn would occur pretty soon. The water in the mouth and eyeballs would evaporate pretty soon. It is unlikely that the blood would boil per se - cell pressure would stop that However, moisture would escape from the skin, throat and lings very fast. The astronauts body would probably become vacuum dried within hours.