Firewalls are software or firmware that enforce policies about what information packets will be permitted to enter or leave a system. Firewalls are arranged across hardware devices to channel traffic and lower the risk of malicious packets traveling from the public web into a closed or private environment. Firewalls may also take the form of standalone software.
The term firewall is a metaphor that describes a kind of physical obstruction used in buildings. Physical firewalls are erected to limit the harm a fire can cause as it travels from one building towards others. In information technology, firewalls are a virtual barrier that is set up to restrict the potential internal harm from an externally sourced cyberattack.
Low-level firewalls are sometimes placed on the external perimeter of a system to inspect traffic, and as they range in severity firewalls can serve as active filters of traffic traveling in both directions. Firewalls can also be set up to protect cloud applications as many enterprises are migrating their resources to hosted or hybrid environments.
"Do you have one or more links in the email that you're having difficulty sending me? If so, our firewall is preventing your email from getting to me. Your email is out of our domain and the links are flagged because we receive many malware attempts."