The presentation layer is the sixth level of the seven layer OSI model. It responds to service requests from the application layer and issues service requests to the session layer.
The presentation layer is responsible for the delivery and formatting of information to the application layer for further processing or display. It relieves the application layer of concern regarding syntactical differences in data representation within the end-user systems. Note: An example of a presentation service would be the conversion of an EBCDIC-coded text file to an ASCII-coded file.
The presentation layer is the first one where people start to care about what they are sending at a more advanced level than just a bunch of ones and zeros. This layer deals with issues like how strings are represented - whether they use the Visual Basic method ("13,thisisastring") or the C++ method ("thisisastring/0"). The ideal is that the application layer should be able to point at the data to be moved, and the Presentation layer will deal with the rest.
Encryption is typically done at this level too, though it can be done at the Transport or Network layer for higher security. Another example is representing structure, which is normally standardised at this level, often by using XML. As well as simple pieces of data, like strings, more complicated things are standardised in this layer. Two common examples are 'objects' in object oriented programming, and the exact way that streaming video is transmited.