I think you can look at it like this TCP - connection centered (without going into nitty-gritty) it sets up a connection between two communicating computers th

I think you can look at it like this TCP - connection centered (without going into nitty-gritty) it sets up a connection between two communicating computers that has "state"; connection setup, connection ongoing, and connection teardown. The data may arrive out of order, but the "receiver" of a stream will rearrange them within a limited window, so there are in order, and acknowledge reception of the packets as part of the protocol (TCP), higher layers like your software generally can rely on it for that. UDP: connectionless, just label the packet with the destination and send it off. No acknowledgement at the UDP layer that the packet was ever received. No order guarantees. The upper layer (usually your software) must manage reception of the packet, ordering, AND request the packet be sent again in those cases where they -must- be received (like a file). Some applications don't require that (say you're talking on a phone call and 1 out of 1000 sound packets drop. It doesn't matter, as the packet is useless if it's outside a few milliseconds of being sent. It's simply dropped, and no request is sent back to try and get it again. I hope I got that helps :)