A network interface card, or NIC, sends and receives data as digital signals, which are compatible with computer technology. A modulator-demodulator, or modem, translates data between the digital format used by computers and the analog format used over phone and cable lines.

Some cable modems are capable of supporting speeds of up to 30 Mbps, although most cable Internet providers only offer packages up to 6 Mbps. NICs are much faster, some boasting speeds of 1 Gbps (1024 Mbps) or more.

Because modems must convert data from analog to digital and often transfer that data to a server many miles away, they are more error-prone than network cards. NICs have a simpler role in networking and are less likely to malfunction.

High-speed cable modems can cost anywhere from $50 to $300, while fax modems are much cheaper at $10 to $50. The price of a network card varies greatly depending on the speed and how many ports it has, ranging from $10 to upwards of $150. (Prices are as of July, 2010.)

NICs have few secondary uses, although some electronics can be powered via an ethernet cable connected to a network card. Fax modems can be used to send and receive faxes and to send and receive phone calls through a modem-equipped computer using available software.