A Firewall put simply keeps stuff from here talking to stuff over there. Firewalls exist in many different possible configurations with both hardware and software options as well as network and host varieties. Most of the general user base had their first introduction to Firewalls when Windows XP SP2 came along with Windows Firewall installed. This came with a lot of headaches, but to Microsoft’s credit it did a lot of good things. Over the years it has improved a great deal and while there are still many options that go above and beyond what it does, what Windows Firewall accomplishes it does very well. Enhanced server-grade versions have been released as well, and have a great deal of customization available to the admin.