what is fire wall in network

A firewall in a car isoltaes the passengers from the heat and potential fire in the engine.

A firewall on a network isolates one network from another. It has a number of rules on it about the types of traffic to allow(and conversely block) between the two networks. It can be a server with 2 network cards and some special software, or a specialised hardware device.

Its most commonly found between corporate networks and the Internet, or home networks and the Internet. More rarely you can find it within corporate networks to protect secure servers from the rest of the company.

James