A useful analogy for understanding the difference between TCP and UDP is the difference between a telephone call and posted letters. The telephone call is not active until the caller "rings" the receiver and the receiver picks up. The telephone channel remains alive as long as the parties stay on the call, but they are free to say as much or as little as they wish to during the call. All remarks from either party occur in temporal order. On the other hand, when you send a letter, the post office starts delivery without any assurance the recipient exists, nor any strong guarantee about how long delivery will take. The recipient may receive various letters in a different order than they were sent, and the sender may receive mail interspersed in time with those she sends. Unlike with the postal service (ideally, anyway), undeliverable mail always goes to the dead letter office, and is not returned to sender.