Layer 6 of the OSI model corresponds to the presentation layer. The presentation layer more or less is aligned to the user concerns. It tries to find solutions for commonly requested problems so that each user need not find his own solution. It services the application layer immediately above it. Presentation layer provides abstraction for the application layer by removing the concerns of the syntax and semantics of the target machine being different.
Suppose we want to transfer a sequence of integers from one host machine to another. As you know, there are various forms of integer representation. If one computer uses a particular representation, then it is possible that the destination machine uses yet another representation. If there is no particular method for providing a common data representation scheme, then data transfer would fail as the target machine will not be able to read the received data.The presentation layer can then convert the received data into a format the host machine recognizes.
Another important function of the presentation layer is encryption. When we want to pass a message securely, we encrypt the message using message encryption techniques and send it over to the target machine. The presentation layer at the receiving end will decrypt the message and pass it on to the session layer. We could also say that compression and decompression is another function of the presentation layer.
Many times, the application layer and presentation layer are not considered separate. One such example is HTTP protocol which is generally an application layer protocol, but has several aspects of presentation layer as well. Currently there are no devices that works on the session layer or the presentation layer.
