OSI Layer Definitions

Physical

the actual "bit path" between two directly-connected communicating "entities". Hardware, in other words.
 

Data Link

Framing and media access control protocols. It's quite useful to have this separate layer.
 

Network

Equivalent to IP, the Internet Protocol. The OSI model actually defined a very similar protocol to IP, called CLNS, as well as several others which were nothing like IP.
 

Transport

Pretty much exactly equivalent to the transport layer in the Internet. The OSI protocol TP4 was functionally equivalent to TCP.
 

Session

no real equivalent in the Internet architecture, this layer was supposed to schedule connections, etc, subject to cost or other constraints.
 

Presentation

the functions of this layer are integrated into the application layer in the Internet. Mainly provides data formatting services. ASN.1 was part of this layer.
 

Application

Very similar in function to the Internet application "layer", although the OSI application protocols were, in general, vastly more complex.