How Giant Tsunamis Work?

They come unexpectedly, grow to the size of a multi-storey building and destroy everything in their path, taking thousands of lives. No, these aren't monsters from popular films. I'm talking about ordinary seawater that suddenly gathers into a tsunami. If you think they'll never concern you, I wouldn't be so sure. How do you recognize such an impending threat? And how does the ocean produce such killer waves?