Lesson 3 - ICND1(1.3a) Use the OSI model and their associated protocols to explain how data flows in a network

The OSI Model has no functional application; it is simply a way we can talk about networks, and helps explain a process called Encapsulation.  Refer to the image below, which shows the different layers of the OSI.  Note that user data flows down from the application layer to the physical layer before it is transmitted to the destination.  When it arrives at the destination, it travels back up from the physical layer to the application layer.  The corresponding layers on both the transmitter and receiver believe they are communicating directly.  Layers below it are transparent.  This process is called encapsulation, which will be explained later.

Diagram of the OSI Model from University of Washington (http://www.washington.edu/lst/help/computing_fundamentals/networking/osi/). A short description of each layer is also available here.

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The following is a very high level explaination of the OSI Reference Model (Exit to Cisco Website):

https://learningnetwork.cisco.com/videos/1125

Part 1 Lesson 1 - Story Time-Analogy for The OSI Ref Model & Review 

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The following explains the levels of the OSI Reference Model in more detail(Exit to Cisco Website): 

https://learningnetwork.cisco.com/videos/1126

 Part 1 Lesson 2 - Framework & Function of the OSI Ref Model & Review 

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The following is a review (Exit to Cisco Website):

https://learningnetwork.cisco.com/videos/1127 

Part 1 Lesson 3 - Review Games - OSI Ref Model 

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Great video on OSI Model from elithecomputerguy via Youtube.
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For a great explanation of encapsulation in regard to the OSI model, refer to the following page (From TCP/Guide)

http://www.tcpipguide.com/free/t_DataEncapsulationProtocolDataUnitsPDUsandServiceDa.htm
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At this point you should have a good understanding of the OSI model.