Mean: $\bar{x} = \frac{1}{n} \sum x_i$
Median: value of data point at $\frac{n+1}{2}$th position
Mode: most frequent value
Percentile: value of data point at $\frac{p}{100}(n+1)$th position
Quartile: value of data point at $\frac{q}{4}(n+1)$th position
Variance: $\sigma^2_x = \frac{1}{n} \sum (x_i - \bar{x})^2 = \frac{1}{n} \sum x_i^2 - \bar{x}^2$
Standard Deviation: $\sigma_x = \sqrt{\sigma^2_x} = \sqrt{\frac{1}{n} \sum (x_i - \bar{x})^2} = \sqrt{\frac{1}{n} \sum x_i^2 - \bar{x}^2}$
Interquartile Range: $Q_3 - Q_1$