Imagine you are a an astronaut working outside the space station in the hard vacuum of space. You are wearing a spacesuit, and it is protecting you against the vacuum. But what if something goes wrong – for example the suit gets a hole sliced in it by an errant tool – and the suit depressurizes? If this video you can see exactly what happens to a human being in that situation:

The wearer of the spacesuit describes a feeling of fizzing on his tongue right before he passes out. This video explains why:

This page from NASA offers more details:

Human Body in a Vacuum

You do not explode and your blood does not boil because of the containing effect of your skin and circulatory system. You do not instantly freeze because, although the space environment is typically very cold, heat does not transfer away from a body quickly. Loss of consciousness occurs only after the body has depleted the supply of oxygen in the blood.

Depletion of oxygen takes about 15 seconds. You might wonder why someone can hold his breath for several minutes on earth, but not in space. On earth, when you hold your breath you have a supply of oxygen in your lungs. In a vacuum, you cannot hold air in your lungs like that because your lungs want to expand like a balloon:

In the video, the balloon expands as the air is sucked out of the bell jar. That same expansion would happen to your lungs if you tried to hold your breath in the vacuum of space (causing extreme lung damage and bleeding in the process). So your body immediately exhales all the air out of your lungs, filling the lungs with vacuum, which then starts sucking the oxygen out of your blood.

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