The OSI model (Open Systems Interconnect) is used to describe the different layers of protocols used in communicating across the network. The OSI model has largely been replaced by the TCP/IP stack, but is still useful for teaching purposes. The model consists of seven layers, and is designed so that changes in one layer will not affect the operation of the layers above and below. The different layers can be remembered by the mnemonic "Please Do Not Touch Susan's Perfect A**." (Sexist, but memorable.)
While this mnemonic works its way up the stack, the model is usually presented starting at the Application Layer, and working its way down to the Physical Layer. This is to reinforce the following idea: Data begins at the application layer on the sending host where it works it way down through the layers across the network before working it way up the layers on the recipient.
The Application Layer provides services to the user, ideally transparently so the user utilizes the network as if it was the host they are using. For example a network drive should be treated as if a local drive despite the intermediate steps in providing it to the user.
The Presentation Layer converts data from applications into a format that can be understood on the network. Encryption is also performed at this layer.
The Session Layer which manages communication sessions by initiating/ending sessions as well as deciding which host can send data and how much at a time.
The Network Layer for reliable communication across the network. Logical addressing such as IP addresses is used at this layer to enable route selection. An example of a network layer protocol is IP. A router is a typical layer 3 device.
The Data-Link Layer this provides transmission of data across medium using MAC addresses, flow control such as packet filtering performed by switches/Bridge. The best known data-link layer protocol is Ethernet.