Modem

Modem 

Modem is a device attached to computers that can convert digital signals to analog signal and vice-versa. Digital Signal is a group of discrete electronic units that are transmitted in rapid succession, and an Analog Signal consists of continous electrical waves. Modem is short name for Modulator - DeModulator where Modulator can convert digital signals to analog signal and DEModulator does the opposite

Modem is a device that enables computers, facsimile machines, and other equipement to communicate with each other across telephone lines or over cable television network cables. In the strictest sense, a electronic equipment (modem) could be a device that converts between analog signals,like sound waves, and digital signals, which are used by computers.

Since, telephone cables can carry analog signals, the computer generated digital signal is first converted into analog form so that it can be carried by telephone lines. Upon receiving it, the modem at receiver end reconverts it into digital form so that receiver computer can understand it . 
Modem transmit data at different speeds, measured by the number of bits of data they send per second (bps). Examples of Modem speeds are: 256 Kbps, 512 Kbps, 2 Mbps, 8 Mbps etc. shows in a modem .