Understanding The OSI Model

[Upper layers]
7. Application:
Provides interface between software applications and network for interpreting applications’ requests and requirements.
6. Presentation:
Allows hosts and applications to use a common language; performs data formatting, encryption, and compression.¬†a.k.a. “syntax layer”.
5. Session:
Establishes, maintains, and terminates user connections.
[Lower layers]
4. Transport:
Ensures accurate delivery of data through flow control, sementation and reassembly, error correction and acknowledgement. (TCP)
3. Network:
Establishes network connections; translates network address into their physical counterparts and determines routing through logical addressing (IP addresses).
2. Data link:
Packages data into frames appropriate to network transmission method.
2a. Logical Link Control (LLC) Sublayer:
Addressing and channel access control mechanisms
2b. Media Access Control (MAC) Sublayer:
Node-to-node error control.
1. Physical:
Manages signalling to and from physical network connections.

Examples of devices/services:
Physical – Ethernet, Token Ring. A hub or repeater works here.
Data Link – Ethernet, ATM. A switch or bridge works here.
Network – ARP, RIP, IPv4, IPv6. A router works here.
Transport – TCP, UDP, SPX
Session – PPTP, H.245, NetBIOS, AppleTalk, WinSock
Presentation – JPEG, GIF, MPEG
Application – DHCP, DNS, FTP, HTTP, LDAP, SSH, SIP, SMTP

References –
http://support.microsoft.com/kb/103884
http://en.wikipedia.org/wiki/OSI_model
http://compnetworking.about.com/od/basicnetworkingconcepts/l/blbasics_osimod.htm