i think it is about priodic nature of frequency domain representation of Discrete time sequences.
they are all periodic with period of 2pi in freq domain. (pi represents nyquist frequency;i.e. maximum available(meaningfull) frequency that a Discrete time signal can carry information about.)
it is widely available in net(i think you can even find it in EDAboard if you search):
Discrete time signal processing by Allan V.oppenheim. you can find your answer in it's few 1st chapters. It's the best reference...