http://www.britain-info.org/faq/xq/asp/SID.260/qx/showfaq.htm
and
This holiday, which is observed on December 26, the feast of St. Stephen, is a unique part of the Christmas season in Great Britain, as well as other Commonwealth nations. Boxing Day stems from a tradition that began in the Middle Ages more than 800 years ago. On this day, English churches would open their "alms box," in which people had placed gifts of money throughout the year, and distribute its contents to needy members of the community. It was also a day for servants to celebrate the holidays with their families, having usually worked the day before. Today, Boxing Day is one of the twenty-two paid holidays received by most working Brits. It is equivalent to a legal holiday or a three-day weekend in America and is often observed on a Monday. Most people now spend Christmas Day with their family and reserve Boxing Day for exchanging gifts with friends.
and someone else says ...
December 26th - is a tradition from WAY back, when many working people were "in service" or apprenticed or otherwise worked away from home and had very little time off. If they were household servants, they certainly couldn't have Christmas day off as they were needed to look after the family and guests. Boxing Day was the day they DID always have off and were allowed to go home to visit their families, taking with them "Christmas Boxes" of gifts.
Nowadays Boxing Day is usually the day when families travel to meet together if they spent Christmas Day at their own home - my brother and his wife always come over and we - that is, my extended family and my mother - join them for a meal.
It is also customary for householders to give small gifts or monetary tips to regular visiting tradespeople (the milkman, dustman, coalman, paper boy etc.) and for employers to give a Christmas bonus to employees, and these gifts are still called "Christmas Boxes."