How are variance and standard deviation used as measures of risk for both a security and a portfolio?

How are variance and standard deviation used as measures of risk for both a security and a portfolio?
Risk reflects the chance that the actual return on an investment may be very different than the expected return. One way to measure risk is to calculate the variance and standard deviation of the distribution of returns.
Consider the probability distribution for the returns on stocks A and B provided below.StateProbabilityReturn on
Stock AReturn on
Stock B
120%5%50%
230%10%30%
330%15%10%
320%20%-10%
The expected returns on stocks A and B were calculated on the Expected Return page. The expected return on Stock A was found to be 12.5% and the expected return on Stock B was found to be 20%.
Given an asset's expected return, its variance can be calculated using the following equation:

whereN = the number of states,pi = the probability of state i,Ri = the return on the stock in state i, andE[R] = the expected return on the stock.
The standard deviation is calculated as the positive square root of the variance.

Note: E[RA] = 12.5% and E[RB] = 20%
Stock A

Stock B