What’s difference for variance and std. deviation calculation for a population vs sample? I just want a Quick overview if one of you have time & patience.

The basic difference is that if you calculate the variance/StD for a sample, you divide the sum of squared distances from the mean not by N which would be the number of all observations, but by (n-1). (N stands for the number of observations in a population, whereas n stands for the number of observations in a sample.) This small correction is done especially in samples with less than 30 observations and it improves statistical properties of the variance.