Porter's five forces

This week in my management class we talked about Porter's five forces. In 1979 a student by the name of Michael E. Porter invented a business strategy that helped determine the attractiveness of a market. The following are the five sources:
1-The threat of substitute products
2-The threat of the entry of new competitors
3-The intensity of competitive rivalry
4-The bargaining power of customers
5-The bargaining power of suppliers

Obviously some of the five forces have more pull than others in specific markets. An example would be the threat of the entry of new competitors in the auto industry. The auto industry is an oligopoly and it would be very hard to enter into the market. Porter's five fources can be a very powerful tool in determining the attractiveness of a market especially to entrepreneurs.