The Transport Layer is the third layer of the OSI and TCP/IP network models, which provides a method of reaching a particular service on a given network node. Transport layers recognize lost blocks of data and request retransmissions from the host computers to ensure that transferred contents are a perfect copy of the original.
"This layer breaks large messages into data packets for transmission and reassembles them at the destination. The two most important protocols in this layer are Transmission Control Protocol (TCP) and User Datagram Protocol (UDP). TCP guarantees delivery of data; UDP just sends the data without ensuring that it actually reaches its destination."