The transport layer multiplexes data for various connections. Each connections represents a path between a pair of applications on a pair of computers. The connections multiplexed by the transport protocol may come from several computers and may be for several appliations.
In the TCP/IP family of protocols, there are 2 transport layer protocols: TCP which guarantees in order delivery of all data sent on a connection and UDP which does not guarantee delivery of all data sent on a connection and does not guarantee in order delivery.