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Population standard deviation (σ)
For a full population of size $n$ with mean $\mu$, the standard deviation is
$$\sigma = \sqrt{\frac{\sum (x-\mu)^2}{n}}$$
Sample standard deviation ($s_{n-1}$)
An estimate of a population’s standard deviation based on a sample of size $n$:
$$s_{n-1} = \sqrt{\frac{\sum (x-\bar{x})^2}{n-1}},$$ where $\bar{x}$ is the sample mean.
Sigma notation (Σ)
A notation meaning “sum of.” For example, $\sum x$ means add all the data values $x$ together.