Most people buy a PC in order to go online. For this reason, the hardware necessary to get onto the internet and to network is of the utmost importance. A modem is a device that lets your computer communicate with other computers over a standard telephone line. This term means that the modem converts the digital information from your computer to an analog signal that can travel over phone lines. Then the receiving modem converts the analog signal back to a digital signal for the receiving computer to work with.
Some modems can exchange data only.
A brief rundown of the AMR slot. Jim does a great and encompassing comparison between several dial-up modems. A quickie tutorial on installing an external modem. A brief discussion of the internal modem. A look at the various modem protocols used since the beginning.