Audio-signals are sent to large distances in radio transmissions. it is clear that the energy of a wave is directly proportional to its frequency. The signal power is very small at audio frequencies (20hz to 20khz). Therefore, we cannot feed audio signal to the anteena for communication purpose.the radiation of electric energy is possible only at high frequencies(.20 khz). the high frequency signals can be sent to thousands of kilometers. even with small power. Hence, before applying the audio signal to the antennawe go through a process called modulation. in this process the audio signal is combined with a high frequency wave called the carrier wave and the question arises that why a seperate high frequency wave is needed ? the answer is that we cannot change any of the charecterstics (amplitude, frequency or phase) of the audio signal as this would change the message to be communicated. So keeping the audio signal same, the amplitude or frequency or phase of the high frequency carrier wave is modified according to the modulated signal. the resulting wave (also known as modulated wave) inherits all charecterstics of the audio signals. this process of superposition of audio signal over carrier wave is called modulation, after modulation this wave can be fed to the anteena and message can be communicated to a large distance. At radio receiver, the audio signal is extracted from the modulated wave by the process called demodulation. The signal is then amplified and reproduced into sound by loud speaker.
A communication system is defined as a set of equipment and facilities that provides a service i.e. the transfer of information between users located a various geographical points.
see the communication system block diagram below.

Transmitter is used to transmit the information using antenna. the channel is a medium and acts partly as a filter to attenuate the signal and disortsit's waveform. receiver is used to collect information at output.