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Sampling is a process of capturing the values of the signal being sampled at regular intervals of time decided by the sampling frequency. Sampling the DC signal will result all the sample values to be the same. All ADCs will have sample and hold circuitry which does the sampling.
You make some coffee, but its too hot. So, you set it down on a table and wait. You touch the cup once in a minute to see if its hot. This is sampling.
The coffee is the signal and you are the sampling circuit. You 'observe' the state of the signal x times a second or a minute or any other duration of time.
The ADC has a circuit called the sample and hold circuit that does this. It captures the present value of the signal and stores it till it is ready for the next sample. This is analogous to the coffee situation where, say you touch the cup at time 0 seconds, then you touch the cup at 60 seconds, 120 seconds, 180 seconds etc. Here, after you touch the cup at 0 seconds, you make the assumption that the coffee temperature wouldn't have changed much and take your next sample after 60 seconds. This is the 'hold' part of sample and hold.