What is the porter's 5 forces model?

Created by a Professor Michael E. Porter, from Harvard, this model explains the various forces applied to a business.

Competition in the industry : Are there competitors in the industry?  If so, are they numerous and weak or is the industry dominated by a few major players?

Potential of new entrants into the industry : What's the risk of having new competition?  If you are selling a product, can you protect it with a patent for example?

Power of suppliers : Can the suppliers of what you need easily affect the prices?  It's basically asking if there is competition in your suppliers' market.

Power of customers : That related to your customer base.  If your customer base is large, chances are no individual will be able to force your price down.  But if you are dealing with a limited number of customers, one of them might force you to lower your prices.

Threat of substitute products: Is there any comparable product/service offered at a lower cost that might bring your prices down?