The fourth of seven layers of the ISO/OSI model.The transport layer defines protocols for message structure, and supervises  the validity of the transmission by performing some error checking.Transport Layer never actually transports the data but only prepares for transporting.The Transport Layer is also responsible for fragmentation and reassembly.

The primary function of this layer is to control the communications session between network nodes once a path has been established by the network control layer.

Responsible for the following :
•Segmentation
•End-to-end Communication
•Flow Control
•Error Control
•Multiplexing of Applications

TCP, UDP work at this layer.