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if a signal which has frequency say from 0 to fm and if we are transmitting it over a channel(or some medium) the channel may have different attenuation /gain for different frequency components and when looked at the receiving end it looks as if the signal is distorted, if we use a block which can ampligy frequecy components which were attenuated by channel/medium and vice-versa in such a way that we can get back the signal properly. The BLOCK which does it is called an equalizer and this technique is called equalization.
It depends what you are going to "equalize".
As mentioned by arunkumar446 you can try to equalize the amplitude resp. the gain of a communication chain - if necessary.
In some cases, however, it is necessary to equalize the delay (which means you are going to linearize the phase) of a system. This can be done with one or even with several allpass circuits.
Many communication channels show frequency depended behavior (for example a multi-path propagation path in GSM/UMTS, etc). As you might know, this may result in spreading of pulses, echo effects, etc. These effects may disrupt demodulation of modulated signals.
Equalization is the insertion of a transfer function that counteracts the effect of the communication path. Of course, the communication path can also be a cable with various connectors in between that gives distortion due to multiple reflection.
As many RF communications path are dynamic, the pulse distortion changes dynamically (as one moves). Good equalizing corrects both amplitude and phase irregularities caused by the communication channel.
Equalization can be done by transmitting a trailer that is known by the receiver. As the receiver knows what it had to receive, the receiver can change the parameters for a FIR filter to obtain the original waveform. These settings are used to correct the data that follows the trailer.
Another method is determining the transfer function from the received trailer and the must be data. By means of inverse fourier transform, one can find the required function to correct the signal.
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