The way I see it is in how it is used in the context of Matthew 5. Jesus is talking about and demonstrating the difference between the letter of the law and the spirit of the law. In the case of verse 28 he says, "Ye have heard that it was said by them of old time, Thou shalt not commit adultery: But I say unto you, That whosoever looketh on a woman to lust after her hath committed adultery with her already in his heart."
We have a habit of doing everything in our power to justify our misbehavior. For example, if I am campaigning for political office I might make a statement such as, "I will never support a tax increase if I am elected." Once I am in office, if I support a new tax would I go against my campaign promise? In the strictest sense of my words the answer is an absolute, "No!" I did not support a tax increase. What I did support was the creation of new taxes. In spirit of my commitment however, I reneged on my promise.
We think that, by the letter of the law or definition of adultery, if we never actually sleep with another person outside of wedlock, we don't commit adultery and therefore we are 'good', but Jesus is pointing out how weak and unjust we are because even the mere thought of lust with another is in violation of the spirit of the law.
I see the 10 commandments in much the same way. We weren't given them as a blueprint. We were given them to show us how weak and evil we truly are and why we need a savior.
Edit: I guess I'm repeating what Danus is saying in a different way. Sorry, I didn't read all the replies before posting.