The IT Stuff

Packet filtering gateways,Stateful inspection firewalls,Proxy firewalls,Personal firewalls,NAT firewalls use the following Access Control Parameters to monitor the traffic
Application:HTTP Data—Kaaza, FTP—abc
Transport:TCP and UDP Port Numbers
Network:IP Addresses, Protocol, Flags
Datalink:MAC Addresses
Policy database(rules table, access control lists, firewall policies)—is a collection of access control rules based on the above parameters

Session layer firewalls are also known as Circuit level firewalls or circuit gateways.
Session layer firewalls operate at the TCP layer of the OSI model.
Session layer firewalls  use NAT (Network Address Translation) to protect the internal network and these gateways have little or no connection to the application layer, thus cannot filter more complicated connections.
Session layer [...]

Can not protect against the inside attacks
Firewalls may represent a significant bottleneck in communication between the protected network and the outside world.
Very little or no effort is taken to look in detail at the data contents of the packet.
Firewalls do not protect against viruses: New viruses are continuously released and there are many ways of [...]

The basic difference between the two technologies lies in how they provide protection for network environments.
Intrusion Detection Systems, IDS, analyze network traffic and generate alerts when malicious activity is discovered.
The limitation of Intrusion Detection Systems is that they cannot preempt network attacks because IDS sensors are based on packet sniffing technologies that only watch network [...]

A firewall is a system designed to secure  your data or control traffic flowing between the Internal Network and the outside Network.

The Packet Filtering Firewall
The Application-Proxy Firewall
The Application-Gateway Firewall
The SOCKS Firewall
Stateful inspection

The Application-Proxy Firewall applies security mechanisms to specific applications, such as FTP and Telnet servers. This is very effective, but can impose a performance [...]