St. Nicholas was a bishop of Myra in the third century. He was born of wealthy parents and used his inheritance to give to the poor.
In many countries, St. Nicholas Day is the day to exchange gifts, Christmas Day being strictly a spiritual occasion.
The tradition of leaving a gift in one's shoe (as well as hanging stockings) comes from the story that St. Nicholas anonymously gave money to a poor man (throwing it through the window and having it land in their shoes or their stockings hanging to dry) so his daughters would not be sold into slavery, but could marry.
St. Nicholas, Pray for us!