The Transport layer is responsible for providing mechanisms for multiplexing upper-layer application, session establishment, data transfer and tear down of virtual circuits. It also hides details of any network-dependent information from the higher layers by providing transparent data transfer.

Services located in the Transport layer both segment and reassemble data from upper-layer applications and unite it onto the same data stream. Some of you might already be familiar with TCP and UDP and know that TCP is a reliable service and UDP is not. Application developers have their choice of the two protocols when working with TCP/IP protocols.

As mentioned above, the Transport layer provides different mechanisms for the transfer of data from one computer to another. Below is a brief diagram which tells you a bit about the protocols. 

These protocols are also analysed in the Protocols section.

Next: OSI Layer 5 - Session Layer