Never really knew the origin myself, but (according to this article) it is an upper/lower class thing as Kent explained.
"The holiday's roots can be traced to Britain, where Boxing Day is also known as St. Stephen's Day. Reduced to the simplest essence, its origins are found in a long-ago practice of giving cash or durable goods to those of the lower classes. Gifts among equals were exchanged on or before Christmas Day, but beneficences to those less fortunate were bestowed the day after.
And that's about as much as anyone can definitively say about its origin because once you step beyond that point, it's straight into the quagmire of debated claims and dueling folklorists. Which, by the way, is what we're about to muddy our boots with.
Although there is general agreement that the holiday is of British origin and it has to do with giving presents to the less fortunate, there is still dispute as to how the name came about or precisely what unequal relationship is being recognized."
Other origins that have been considered are outlined in the full article at:
http://www.snopes.com/holidays/christmas/boxing.asp