Application
Dictates how programs are going to interface with transport layer by session management. Things that operate here are HTTP, Telnet, SSH, FTP, DNS, SSL/TLS, SNMP.
Transport
Defines the level of service / Status of the connection being used by TCP(Connection full), UDP(Connectionless), and RTP(Real Time).
Internet
Data is taken and packaged into IP Datagrams. Examples include IP, ICMP, ARP, RARP(Reverse ARP), etc.
Network Interface
Describes how to transmit bits across a network and determines how the network medium is going to be used. (Ex. What cabling? What are the interfaces used? Token ring or Serial?)