Firewall: Can be a software or hardware device Controls traffic between a system and the internet. All data being sent or received is inspected.
There are two main types of firewall: Packet Filtering and Proxy Server.
1. Packet Filtering: Packets are analysed against a set of filters, if approved they are allowed through, if not they are disgarded.
2. Proxy server: Information from the internet is retrieved via the firewall and then sent to the requesting service. There is no direct contact between the system and internet. The host of a web page doesn't come in contact with the user it comes in contact with the proxy server.