I really like Aet's definitions. Betraying the Truth is a damn good descriptive for lies which will only end up in hurting someone else. Those are the worst kind of lies, and should never be told.

There are also 'white lies' which I actually feel are socially neccessary. Times when telling someone the truth would cause unneeded pain. Aet's example above is a very extreme version of that. It can even be as simple as: "Do you like this dress on me?" Saying yes, if they're happy with the way that it looks, even if you don't like it spares them getting their feelings hurt, and their self esteem battered, by what is after all nothing more than your opinion.