In the field of corporate finance, we are taught to focus on both expected returns and risk. There are two popular risk measures: (sigma)

In the field of corporate finance, we are taught to focus on both expected returns and risk. 

There are two popular risk measures: σ (sigma) and β (beta). (A) What does each "symbol" measure? (B) Theoretically, is β superior to σ as a risk measure? Discuss?