Physical layer – Defines how bits are encoded for transmission over a physical medium. Example: ITU-T V.90.
Link layer – Defines how devices connected to a physical layer share access to the physical medium and exchange data. Examples: Ethernet (IEEE 802.3), Wi-Fi (IEEE 802.11).
Network layer – Defines how data is routed between devices that may be connected to different link layers. Example: IP (RFC 791).
Transport layer – Defines how devices connected to a network layer create and manage communication sessions. Examples: TCP (RFC 793), RTP (RFC 1889), TLS (RFC 2246).
Application layer – Defines how applications provide services and exchange information over a transport layer. Examples: SMTP (RFC 821), HTTP (RFC 1945).
Social layer – Defines how the information exchanged by applications refers to human beings, their activities and relationships. Example: A telephone number, a bank account, a Facebook profile and an email address are distinct identifiers that may refer to the same human being.