Central tendency measures three different things. Mean (arithmetic average) is sensitive to all values, so a single outlier can shift it dramatically. Median (middle value when sorted) is robust to outliers - half the data lies above and half below. Mode is the most-frequent value, useful for categorical-like data or detecting bimodal distributions. For symmetric, normally-distributed data, all three coincide; for skewed data they diverge meaningfully. Indian household income, for example, is right-skewed: mean is far higher than median because a few very high earners pull the mean up.
Dispersion measures the spread. Range (max − min) is simple but sensitive to outliers. Variance averages the squared deviations from the mean - squaring makes positive and negative deviations both contribute and amplifies the influence of large deviations. Standard deviation is the square root of variance, returning to the same units as the original data, making it directly interpretable. IQR (interquartile range, Q3 − Q1) captures the spread of the middle 50%, robust to outliers.
Population vs sample distinction: when your data covers every member of the group of interest (e.g., all 30 students in your class), use population stats - divide by N. When the data is a random sample from a larger population you want to estimate (e.g., 500 voters polled to estimate election results), use sample stats - divide by N−1. The N−1 correction (Bessel's correction) accounts for the loss of one degree of freedom when the sample mean is used as the estimate of the population mean. Wrong choice gives biased standard deviation estimates.