A modem modulates outgoing digital signals from a computer to analog signals for transmission over a pair of telephone lines and demodulates the incoming analog signal and converts it to a digital signal for the digital device. In simple terms, a modem is a device for connecting the computer to the internet.
Types of Modems:
Dial up Modems: These modems are used for connecting to internet over usual telephone lines. These modems are available as internal modems and external modems. The internal modems come as PCI cards and are inserted in a PCI slot on the motherboard. The internal modems are cheaper than external modems, which are installed outside the CPU.
Cable Modem: A cable modem is a device that enables connection to the internet over a local cable TV line. The data transmission speed of these modems far exceeds that of the prevalent 28.8 and 56 Kbps telephone modems.
ADSL or DSL Modems: transceiver, also known as a ADSL (asymmetric digital subscriber line) modem or DSL (digital subscriber line) modem, is a device used to connect a single computer to a DSL phone line. DSL modems use much higher frequencies than voice frequencies for data transmission, in order not to interfere with voice transmission. The data transmission speed is usually much higher than that of dial up connections.
The unit for measuring speed of transmission of data by a modem is bps (bits per second).