why do not we say that pulse of human nerve system as discrete signal?
Because a nerve's pulse is continuous in time and amplitude,
see here for an example.
could some give me example for discrete signals and digital signals?
In the macroscopic world that we deal with in eletronics, there is no real discrete signal. For the purpose of simplification and mathematical/logical treatment signals are very commonly called discrete but mainly to differentiate them from truly analog signals.
Take for example an analog-to-digital converter. You'd say that the input is analog in time and amplitude, whereas the output is a sequence which is discrete in time (controlled by tha clock of the ADC and amplitude, namely '0' and '1'.
But: the output signal is not really discrete in time. For one there is an output voltage at any time and the change in output happens with a clock edge where the edge itself is not 100% fixed in time but has typically slight variations called jitter. The output voltage is considered logic '0' if it is below a defined threshold and logic '1' if it is above a certain threshold. But depending on parameters like e.g. stability of the power supply, a logic '1' can be e.g. 4.5V or 5V or even 5,5V in a nominal 5V system. Any voltegae level in between is possible, the signal is still considered to be logic '1' and therefore in a sense discrete, but not physically.
Also no signal can change instantly between '0' and '1'. There is always a transistion including all voltages between these two states. If you use a fast enough oscilloscope, you can see these edges as ramps.
Note that this is a low level physical answer - you were expecting geeky answers, weren't you?
For many practical purposes you can consider digital signals as discrete, but the higher the clock rate, the more the analog nature of even digital signals becomes noticeable (cf. effects like ringing on transmission lines etc.).