What is the function of a Firewall in a network environment?

"A Firewall is a network security system that protects computer networks from malicious or unauthorized access. It can also be configured to allow limited access to specific users. A firewall consists of a hardware device or software program or a combination of these. All the data that encounters the firewall are scanned against specified security criteria. Data packets meeting the criteria are allowed through the network, and those that don't meet the criteria are blocked. Firewalls can be configured and updated as Cisco's needs evolve and help to provide control over the network's access and security features."