The transport layer provides transparent transfer of
hosts. It is responsible for end-to-end error recovery and flow
control. It ensures complete data transfer. In the IP protocol Stack
this function is achieved by the connection oriented Transmission
Control Protocol (TCP) or the datagram type User Datagram Protocol
(UDP). The purpose of the Transport layer is to provide transparent
transfer of data between end users, thus relieving the upper layers
from any concern with providing reliable and cost-effective data
The transport layer turns the unreliable and very basic service provided by the Network layer into one worthy of the term 'Communication'. There is a long list of services that can be optionally provided at this level. None of them are compulsory, because not all applications want all the services available. Some can be wasted overhead, or even counterproductive in some cases.
Connection Oriented. This is normally easier to deal with than Connectionless models, so where the Network layer only provides a connectionless service, often a connection oriented service is built on top of that in the Transport layer.
Same Order Delivery. The Network layer doesn't generally guarantee that packets of data will arrive in the same order that they were sent, but often this is a desirable feature, so the Transport layer provides it. The simplest way of doing this is to give each packet a number, and allow the receiver to reorder the packets.
Error 'Free' Data. The underlying network may well be noisy, and the data received may not always be the same as the data sent. The Transport layer can fix this: typically by providing a checksum of the data which detects if there has been a glitch of some kind. Of course, error free is impossible, but it is possible to substantially reduce the numbers of undetected errors. This layer may also retransmit packets which have gone missing en route.
Flow Control. The amount of memory on a computer is limited, and without flow control a larger computer might flood a computer with so much information that it can't hold it all before dealing with it. Nowadays, this is not a big issue, as memory is cheap while bandwidth is comparatively expensive, but in earlier times it was more important. Flow control allows the receiver to say "Whoa!" before it is overwhelmed. Sometimes this is already provided by the network, but where it is not, the Transport layer may add it on.
Byte Orientation. Rather than dealing with things on a packet-by-packet basis, the Transport layer may add the ability to view communication just as a stream of bytes. This is nicer to deal with.
Ports. Ports are essentially ways to address multiple entities in the same location. For example, the first line of a postal address is a kind of port, and distinguishes between different occupants of the same house. Computer applications will each listen for information on their own ports, which is why you can use more than one network-based application at the same time.
On the internet there are a variety of Transport services, but the two most common are TCP and UDP. TCP is the more complicated, providing a connection and byte oriented stream which is almost error free, with flow control, multiple ports, and same order delivery. UDP is a very simple 'datagram' service, which provides limited error reduction and multiple ports. TCP stands for Transport Control Protocol, while UDP stands for User Datagram Protocol.
Some things, such as connection orientation can be implemented at either Transport or Network layer. The idea is that the Network layer implements whatever set of options is easiest: for some underlying networks it is easiest to implement connectionless communication, while for others it is easiest to implement connection oriented communication. The Transport layer uses this simplest set of options to implement whatever combinations of options are actually desired.