Network security is one of the more technical subjects of an information security program but it is essential to your overall security health. Your network is your pathway to all of the essential business processes that happen to and from the outside world. The same connectivity that enables business also comes at a cost increased risk of suffering an information security incident if you do not implement firewall protection to prevent undesired traffic to your network.
Technically a firewall can be either a hardware appliance or software that resides on a machine but for our purposes I will assume you plan to utilize a hardware appliance type firewall. The type of firewall that you should choose depends on the size of your organization and your protection requirements. That being said the general principle is that any firewall that is properly configured is better then none.
Some of the leading providers of appliance based firewalls include Cisco, Juniper, Check Point, and SonicWall. All of these companies offer models that can meet the needs of smaller operations all the way to large enterprises. A smaller company (without a present firewall) that gets Internet from a cable or dsl connection should consider an integrated wireless router/firewall model. These are often the same models utilized by home users and serve the purpose of separating the network from the Internet at an affordable price. Some of the vendors that specialize in this market include Linksys, Dlink, Netgear, and 3Com.