A personal firewall is used to protect a single host or a small network such as a home network. Home users need to use firewall to protect their system from outside network.
Using a separate firewall system would be expensive. To tackle this problem personal firewall are used.
A personal firewall is an application program that runs on a work station to block unwanted traffic from network. It can complement the work of conventional firewall by screening the kind of data a single host will accept. It may be used to enforce certain policies.
Any of the packet filter, stateful filter or application proxy can be used which are simpler for the sake of efficiency.
Example: Users may decide that certain sites such as computers on company network are highly trustworthy but most other sites are not.
It is general practice to combine virus scanner with a personal firewall. A user may forget to run virus scanner daily. The combination of virus scanner and personal firewall is both effective and efficient.