This example explains how week numbers of year 2009 vary according to different definitions. January 1, 2009 is a Thursday.
When you select First week starts on January 1, the year has 52 weeks, counting from December 29, 2008 to December 27, 2009.
When you select First week starts in first 4-day week, the year has 53 weeks, counting from December 29, 2008 to January 3, 2010.
When you select First week starts in first full week, the year has 52 weeks, counting from January 5, 2009 to January 3, 2010.