A firewall is a tool (usually a combination of hardware and software) that secures access to a local network or computer. A firewall guards the local network by analyzing the data leaving and entering, allowing authorized traffic through while keeping unauthorized traffic out. 

Firewalls fall into several different categories.  Packet-filtering firewalls classify traffic based on a packet’s source, destination, port or other basic information.  More advanced stateful packet filtering firewalls have access to more information from which to make their decisions.  Proxy firewalls, which look at content and can involve authentication and encryption, can be more flexible and secure but also tend to be far slower.