Between the Application Layer and the Network Layer lies the Transport Layer. The job of the Transport layer is to pass data between the Application Layer and the Transport Layer using TCP or UDP protocols.

UDP lives here but TCP is what makes the Transport Layer famous. TCP unlike UDP uses a virtual connection to make sure that the data arrives at its destination intact and in order. TCP accomplishes this "connection" via handshaking and special codes in each TCP data segment.

TCP and UDP receive data from the application and form segments or packets (datagrams) respectively. A destination address is added before passing the packet or segment to the Network Layer.