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Aliasing occurs when a signal is samples in which there are frequency components above half the sampling rate (see Nyquist rate). What happens is that these frequencies are undersampled and therefor indistinguishable from frequencies below half the sampling rate (for example an input Frequency of 30kHz with a sampling rate of 44100 becomes a frequency of 44100-30000=14100Hz when the signal is reconstructed). This is similar to the effect you see when something that is rotating very fast is filmed (it can appear to be rotate backwards or very slow for example).

Additional reading:

Aliasing example this is a java applet designed to illustrate the concept of aliasing of a sinosudial signal via visual approach.