This technical phrase is frequently heard on Star Trek when a starship is getting hit hard in battle. Theoretically, in Sci-Fi, explosive decompression is where the parts of the hull of the ship are exploding into space rapidly, and air is quickly venting from that section. In the real world explosive decompression is simply rapid decompression. This is very different than slow decompression, which would be a small leak affecting the air pressure and eventually starving the spacefarers of oxygen. Slow decompression occurred onboard Mir now and then.
Geoffrey A. Landis has written an excellent paper describing the effects of explosive decompression and vacuum exposure at http://www.sff.net/people/Geoffrey.Landis/vacuum.html
If at some point you are about to experience rapid decompression, do not try to hold your breath! Your lungs will explode. People exposed to sorts of explosive decompression may develop a painful and often fatal disease known as The Bends.
Editor's note: As outlined by Lucy-S at SFF Net, the site referenced above has gone offline. As of March 2017, you can find the referenced article at www.geoffreylandis.com/vacuum.html.