Before you can contemplate contamination, you have to get a little heavy and define life in a strictly biological sense. What is it? Is the organic life we see on Earth the same kind we can expect on a planet in another galaxy?
Well, in the solar system immediately surrounding our home planet, life probably does obey similar biological and physical principles. If Mars, for example, possessed an Earth-like atmosphere and liquid water billions of years ago, then you might expect that carbon-based life forms might have evolved there, too. Indeed, some scientists speculate that life on Earth came from Mars (the ultimate example of planetary contamination!). The idea is that meteorites knocked loose from our red neighbor traveled across space and smacked our young, just-developing planet. These meteorites might have carried the "seeds" of organic life, which nestled into Earth's warm, watery bosom and began the evolutionary journey to produce the vast diversity of species we know today.
Another important development in defining life has been the study of weird and exotic organisms on Earth. Biologists refer to these creatures as extremophiles: organisms that thrive in extreme conditions, such as strong acid, low oxygen or extremely high temperatures. Apparently, Dr. Ian Malcolm, the wry mathematician in "Jurassic Park," had it right when he said, "life finds a way." There may be no place on this planet, even environments poisonous to higher organisms, where highly specialized microorganisms can't live quite comfortably. And if life finds a way in Earth's extreme environments, then it stands to reason that it could do the same in the harsh conditions found on Mars or even Venus.
This logic forms the foundation of planetary protection and drives its two main priorities: to prevent forward and back contamination. Forward contamination occurs when Earth-based microbes hitch a ride on a NASA rocket (or a NASA astronaut), land on another body in the solar system and, once there, decide to stick around. In fact, to a hardy microbe, Martian soil represents just one more extreme environment to which it must adapt. The reverse could happen just as easily. In back contamination, an extraterrestrial bug, hunkered down in the barren soil of its home planet, could attach to an astronaut's boot, journey to Earth and start living large in its new, five-star resort.
NASA designs its planetary protection program to prevent either type of contamination. How it manages that awesome feat is up next.