What is a lie in God's eyes?

So me and my friend were wondering about this during our bible study time at lunch. What is a lie in God's eyes? Say you know something that would cause a problem for your friend, and you hope no one is ever going to ask you if you know anything about it. In order to protect your friend from ill-willed people, do you lie and say "I know nothing about it"? Or maybe you should answer "I don't feel comfortable talking about that person". Or even more so say "I can't share that with you". Are you always supposed to speak the truth, even if it means that someone you love can get hurt? I find that very tricky. What if it is your direct supervisor? You are under their authority, so you are technically supposed to honor that. Just from a Christian point of view. Then we got reading 1 Samuel.... about Jonathan and David and how Jonathan protected David by misleading his own father and basically spying out his fathers plan to kill David. He then tipped off David to keep him safe. Does that constitute as lying? Deceiving? Plotting? Did he hear directly from God to keep David safe? Hmmmm, I am intrigued. So we decided to read all of Samuel to get a better understanding of what was happening. We are still working on it. So many questions......