Internal modems are boards which are literally plugged into the motherboard inside the computer case. The boards may be short and occupy only half of a typical motherboard slot or long and occupy a full motherboard slot. A typical internal modem usually includes an on-board speaker, which is software adjustable for monitoring the progress of calls, and two standard telephone jacks for connecting a telephone line and a telephone instrument. External modems are boards which are mounted in a separate case and connected to a computer's communications port by a cable. External modems generally contain an on-board speaker and two standard telephone jacks.
External modems are generally more expensive than an internal modem.
The speed at which a modem operates is measured in baud. A fast modem can greatly reduce the time required to transfer long computer files thus reducing telephone and information service connect charges. Generally all modems are downward compatible, i.e., a 9600 baud modem can be slowed down to connect to a 2400 baud modem. You should buy the fastest modem that you can afford. Major makers of modems are Hayes and U.S. Robotics.