TCP is a guaranteed delivery system - it waits for acknowledgement of the packet being received before sending the next packet. If it fails to receive acknowledgement it will keep attempting to resend the same packet. UDP is not a guaranteed delivery system. It will keep sending the next packet until there aren't anymore packets to send. Generally, TCP is used in data communications and UDP is used for things like streaming audio and video. The human experience is better with a millisecond of missing audio than it is pausing to "buffer", especially with interactive things like telephone conversations and video chats. After all, wouldn't you rather miss a half-second of your video chat as opposed to having the entire chat fail?