What are some of the factors that determine the supply of labor in a market?

The demand for labor is said to be a “derived” demand.  What is the meaning of a derived demand? How does this concept help to determine the demand for labor?What are some of the factors that determine the supply of labor in a market? What significant factors have changed the supply of labor over the last twenty years? How does a firm determine its prices and the quantity of labor required in the resource market during a specific period?