Business Cycle, Recession, Depression
The business cycle is a process of the economy whether the economy is in an expantionary or contractionary phase depending on the growth rate of the economy or real GDP.
Recession is the contractionary phase of a business cycle, it is when there is negative economic growth for 2 consecutive quarters. The economy is having very little or no growth the unemployment rate is high and prices are low.
A depression is when the contractionary period or the economy has not grown or decreased for an extended period of time or a few years. The unemployment rate is extremely high and people are not able to spend money to boost the economy.

Morris,Sayre.Principles of Macroeconomics sixth edition,2009
Amadeo,Kimberly.What is an economic depression?,About.comUsEconomy,March 7 2012,
http://useconomy.about.com/od/grossdomesticproduct/f/Depression.htm