An Application Level Firewall is a firewall where one application-level (i.e., not kernel) process is used to forward each session that an internal user makes to a network resource on the public network.
Application Level Firewall's are considered to be the most secure type of
Firewall's, but they incur a significant performance penalty.
The penalty arises because a new process must be started each
time a user starts a new session -- for instance by following
a URL to a new World-Wide Web site.