An external modem is a stand-alone unit connected to your computer with a serial cable. Another cable connects the modem to a telephone jack. You must also plug the modem into an electrical outlet. Internal modems are more likely to be found in portable computers. A cable from the back of the computer plugs directly into a telephone jack.
Internal modems do have disadvantages.
Moving the modem to a different machine is tedious. Resetting an internal modem usually requires rebooting the machine. Troubleshooting is more difficult and requires that you take the entire computer to the shop. An internal modem permanently occupies one serial port on your computer - you cannot easily remove it temporarily to plug some other device into that serial port. Easily inserted and removed, these have the convenience of internal modems for travelers, and the benefits of external modems for troubleshooting.