tcp stands for transmission control protocol. it is more reliable than its udp counterpart. tcp has a much larger header length in order to accommodate check sums, etc. udp stands for user datagram protocol and is much less reliable. it's used more in applications that are time sensitive like web-camming. for example, if I dropped a packet at some time during our facetime chat, i probably wouldn't care about it in the future. lots of applications use combinations of both protocols