Here is a very brief overview of the seven network layers.
Layer 1 (Physical Layer)
An ethernet hub is completely in Layer 1.
Layer 2 (Data Link Layer)
An ethernet switch is partly in Layer 2 as it must have knowledge of the Media Access Control (MAC) address of packets.
Layer 3 (Network Layer)
The funtion of routers is at this level.
This is the IP layer.
Layer 4 (Transport Layer)
Ports are defined at this level.
TCP, UDP, etc reside at this level.
Layer 5 (Session Layer)
Is a conseptual layer describing sessions.
Layer 6 (Presentation Layer)
The Secure Sockets Layer (SSL) protocol is defined at this level.
HTTP compression is also handled at this level.
Layer 7 (Application Layer)
Hypertext Transfer Protocol (HTTP) resides at this level.