Explain OSI layers and TCP/IP in layman's terms?

The best way to do this is with an analogy. So, to review, the 7 layer OSI stack is as follows:


 •              Physical
•              Data-link
•              Network
•              Transport
•              Session
•              Presentation
•              Application
           

This way of looking at networking can be applied to nearly any form of communication. So let's apply this to exchanging snail-mail letters with a pen pal (yes, this is SO 1977):

  • Physical -- The USPS' trucks, trains, and planes: this is how the letters actually get from point A to point B.
  • Data-link -- The envelope: you can't just put a handwritten letter in a mailbox and expect it to be sent somewhere.
  • Network -- The address: the USPS needs to know where to deliver the letter. This establishes a connection between two residences.
  • Transport -- Your name on the envelope: once it gets inside your house, it needs to be given to the correct person.
  • Session -- The standard letter format: this includes dating the letters, saying "dear so-and-so" and "yours truly."
  • Presentation -- The body of the letter itself: let's make sure both parties are writing in English.
  • Application -- The collection of letters exchanged: the point of the previous six layers was to enable the pen pal relationship between two people.
Mind you, this analogy--like all analogies--does have some weaknesses, but the OSI 7-layer stack has always been roughly applied to actual networking technology anyway. It's a way of framing discussion, not a real technology to be strictly implemented.

 Cisco_OSI_Layer

OSI layers
OSI layers