I would say that ommission of truth (when you know the person wants to know something) is a form of a lie. But I don't consider all lies bad. For example, I may think that my wife's cooking tonight was horrible. However, I don't consider it a bad lie to tell her I enjoyed it. However, if she was trying to improve her cooking and I lied, then I think it was bad.
One common problem I hear is "I didn't want to tell my wife about my girlfriend because it would hurt her and make her feel bad." I tend to think this is a bad kind of lie (unless she has said that she doesn't want to know about this stuff).