Hey there,

Physical
The physical layer is pretty much the most straight forward because it consits of the phsical networking components such as the Cat5 cable or phone line.

Datalink
The datalink acts as the organizer of the system. It gives error control when frames are missing/damaged. It also regulates the speed of the data transmission.

Network
The network layer is responsible for the control of the subnet. It is the part that controls the route for the packet being sent. It also keeps up with how much data is sent.

Transport
Pretty much the main function for the transport layer is to recive data from the session layer and break it up into 'pieces' and pass it onto the network layer and make sure the 'pieces' arrive at the destination alright. It also is responsible for establishing the correct network connection as set forth by the session layer. A good example of a protocol that uses the transport layer is TCP.

Session
The session layer establishes a session between two computers. The main purpose of this layer is to enable syncronization so thatcollision doesn't happen through transmission.

Presentation
This layer is used to format information being sent or recieved by the two computers according to preferences. An example of this is an EBCDIC text file is converted to an ASCII file.

Application
This layer allows communication between two different apps on two different machines, this layer deals with file name conventions and protocols used on different systems and provides conversion between the two if needed.

I know this isn't very in-depth, but I hope it has helped you a bit.

<edit>Sorry Relyt! When I began my post only Korp had posted.</edit>