1. firewall

2. Firewall Prevent unauthorised access
Just explains what it does + not how it does it

3. First generation packet filtering 1.0
Static packet filtering
each packet entering or leaving the network is checked and either passed or rejected - set on user rules
Source IP address
Destination IP address
TCP/UDP source port
TCP/UDP destination port

4. Stateful packet inspection 1.1
This approach examines the contents of packets rather than just filtering them
Permit outgoing sessions while denying incoming sessions.

5. Second-generation firewalls: Proxy services
increase the level of security between trusted and untrusted networks.
Known as application proxy or gateway firewalls
An application gateway firewall uses software to intercept connections for each Internet protocol and to perform security inspection. It involves what is commonly known as proxy services.
The proxy acts as an interface between the user on the internal trusted network and the Internet. Each computer communicates with the other by passing all network traffic through the proxy program. The proxy program evaluates data sent from the client and decides which to pass on and which to drop

6. Firewalls evolved: The third generation
state-of-the-art perimeter security
These systems  alert administrators in real time about suspicious activity that may be occurring on their systems
New name given - stateful multilevel inspection, or SMLI
screens the entire packet, OSI layers 2 through 7, and rapidly compares each packet to known bit patterns of friendly packets before deciding whether to pass the traffic