1. Signal only in certain discrete function of the instantaneous value is as discrete-time signal. It analog signal from the sample, or is not the time and the magnitude of the added edge discrete signal.Discrete signal analysis and processing, usually by a certain time in order not for a group of pool called "sequence."
2. Two different sequences can be the same as the corresponding Z transform, and the convergence domain is not the same, therefore, To sequence and Z transform is uniquely presented in sequence Z transform, we must designate their convergence domain.
3. In discrete-time signal analysis and processing, often to the sequence together, multiply, and delay operations such as convolution, Z-transform the properties for simplified operation very useful.