A modem is a communication device that allows for transfer of data between two computers at different places. A modem modulates the analog signals to digital signals for the digital device and demodulates the digital signal to analog signal for the data to be transferred through the telephone line or a co-axial signal.
The speed at which the modems operate varies from 14.4 Kbps to the 56 Kbps. Older modems shipped were either 14.4 Kbps or 28.8 Kbps, but most modems in use are 56 Kbps. Other types of modems like the cable modem allow transfer of data at over 1 MB per second.