Ever since internet connections became available, firewalls have been the primary tool to safely connect your internal network to the internet.
Firewall specifics have changed dramatically over the years. Early firewalls were port-blockers, preventing webservers to be accessed through the FTP protocol. Lately, firewalls have developed into strong tools for Intrusion Prevention. Analyzing content and user intentions are a main objective these days.
Most firewalls are combined with VPN technology to ensure safe connections of networks. Technologies to be considered are Network Address Translation (NAT), authentication, URL filtering and even antivirus and antispam.
The latest trend in firewalling is organizing selective access around users and their applications.
Also, these days, considering our day-to-day dependency upon internet facilities, high availability is key. Therefore redundant firewalls are engineered, oftentimes combined with redundant internet connections as well.