Modems, Routers and Firewalls 

Modems
Modems transfer data from one computer to another via a phone line, whether it be direct data transfer from your computer to an associates or from your computer to an ISP's computer giving you access to the internet.  There are two types of modem available 56kbps and broadband the latter being the faster.

Routers
A network device that connects two similar networks having the same network protocol. It also chooses the best path between two networks when there are multiple paths. Used in conjunction with a broadband modem to create mini-networks connected to the internet. This kind of setup may also incorporate a hardware firewall for network security.

Firewalls
A firewall is a hardware or software solution to enforce security policies. In the physical security analogy, a firewall is equivalent to a door lock on a perimeter door or on a door to a room inside of the building - it permits only authorized users such as those with a key or access card to enter. A firewall has built-in filters that can disallow unauthorized or potentially dangerous material from entering the system. It also logs attempted intrusions.