By Cecilia Twanmo
When a computer is connected to internet, it is sent through something called a Modem. A modem, or modulator-demodulator, sends data digitally by modulating it. The modulated data is then sent to a phone line, which is then demodulated (turned back to digital data) by the receiving modem on the other line. This process allows an understanding between the computer, which speaks a digital language, and a phone line, which speaks an analog language. The modem is like a translator.
A modem consists of two types of operating systems which translate: a modulator and a demodulator. The modulator simply transforms the waveform of the data to transmit it. By doing this, the modem takes digital data from a computer and translates it into analog form to be sent through the telephone lines. The data is then received by the demodulator which translates it back to digital form. Once the data is ready to be sent back through the telephone lines, it is translated back to analog form by the modulator. The now analog data is sent back through the telephone line to the original modem, which demodulates the data back to digital form.
There are several different types of modems. The first kind was a simple dial-up modem. This acted quite like a telephone line. The modem would first open the line, then dial a number and wait for the modem a the other end to reply; just like how one would wait for a friend to pick up the phone. Dial up modems are useful today because they communicate with fax machines as well, however one cannot talk on the phone while connecting to the internet. Today, modems are much quicker and advanced. Many laptops for example carry their modem inside them and have WIFI including in them. This way, one would not need a modem to connect to the internet at all, for it is passed through WIFI or another ethernet connection. Other computers use an external modem as one of their peripherals which plug into the computer and into its motherboard. These are connected to the computer by a cord which plugs in to the computer and connects the modem into the wall or cable box. These are considered "high speed" internet connections.
http://computer.howstuffworks.com/modem1.htm