Application Layer Firewall

In simple, the firewall that works at the application layer is considered as application layer firewall.
As the firewall is placed at application level, the administrator can block the inappropriate content such as websites, spams and viruses.

If TCP/IP and OSI reference models are compared, all the top 3 layers in OSI (Application, Presentation and Session) are considered as only 1 layer in TCP/IP(Application). So the Application layer does not route traffic on the Internet layer (which is done by the other layers in OSI). Only the traffic which satisfies the so called rules by the administrator can be put through the firewall.