When the signals are sampled at the rate less than Nyquist rate ( sampling frequency > 2W ), then aliasing take place. [ The sampling rate of 2W samples per second, for a signal bandwidth of W Hz, is called the Nyquist rate; its reciprocal .5W (measured in seconds) is called the Nyquist interval.] Frequencies higher than 'W' takes the form of lower frequencies in sampled spectrum. This is called aliasing.
Aliasing can be reduced by sampling at a rate higher than Nyquist rate.