Standard Deviation, also referred to as volatility, measures the degree of variation/uncertainty of returns around the mean/average return. The higher the volatility of the investment returns, the higher the standard deviation. That is why the standard deviation is often used as a measure of risk.
The standard deviation is available in different variations: monthly or annualized, calculated over different monthly or annual periods.
We use the standard deviation of the SAMPLE as opposed to the standard deviation of the POPULATION. The sample standard deviation is the industry standard.