I agree with Cody and Dimo. I believe the key to Utilitarianism is assessing the possible outcome of your actions, and weighing their various effects as far as you reasonably can. The limits of what would still be a reasonable conclusion ("is it reasonable to conclude that two lives are worth more than one in the given case?") could be established in various ways, such as by another specific system of ethics. There will always be large unknown variables.
The opposite is anything that precludes any weighing at all, such as cast-iron principles ("I will not take another's life, no matter what") or ethics solely based on intentions that are unrelated to results ("if you do it for God, it doesn't matter what you do; if you do it with the intention of pleasing God, you may kill as many as you feel is necessary").
However, it could be said, which Dimo hints at, that all ethics unwittingly work within a Utilitarian framework, rendering the term meaningless: if pleasing God is the highest goal in ethics, because that is my only moral obligation, I can weigh the results of my actions according to that model. Shall I destroy Timbuktu or not? I know God commands me to do it, and so Timbuktu's destruction is the moral thing to do. Not destroying it weight zero.
Thus it would seem reasonable to include in the definition of Utilitarianism some more specific goals, like people's physical well-being as a positive weight, or the fulfilment of their desires. But even these two goals are controversial among Utilitarians.
In practice, whenever I think "your decision is impractical and could profit from a more Utilitarian point of view", I just want to tell people, "think more about the practical results of your actions, instead of these mere principles; you have adopted them seemingly at random, and you are unwittingly giving them far too much weight in your narrow-minded motivation, forgetting the true purpose for which you have invented those principles in the first place".