Throughout the years, the image of the modern day Santa Claus has been cobbled together through the folklore of various cultures, including English, Greek, Dutch, Belgian, and German. The tradtions we associate with Christmas are a combination of Christian and Pagan ideology, fusing together the birth of christ and the pagan traditons of Yule.
Prior to the 20th century each of these cultures and rituals had their own visual interpretation of Santa. With the proliferation of mass media, and with the assitance of 20th century advertising, that image coalesced into the Santa we know today: Red Suit, Big Belly, White beard, etc.