| Many of the legends and customs surrounding Santa Claus may stem from practices that precede the Christian era.
The prsctice of coming in through the roof and leaving items around the fire is likely to have its origins in the Midwinter celebrations of some of the northern peoples, where the Shaman of a tribe would lower him/her self down through the smoke-hole of an underground winter dwelling and leave carvings of herd and game animals around the fire, to release the spirits of those animals to ensure good breeding and hunting for the next year.
The origins of him residing at the North Pole and his use of reindeers may come from the Lappish & Sami peoples, who come from the regions where this was practiced and mould their year to the movements of the reindeer.
As for the red and white costume he wears, some 18th to 19th century depictions often show him in either green and white or blue and white (and less commonly in yellow and white). His costume becoming fixed as the more commonly known red & white probably does come about in the way described on the site mentioned by Fr_Chuck.
If you look at some of the other traditions that are stilled followed at Christmas/Midwinter, you will find that many of them have pre- Christisn origins. But they are answers to other questions. So I'll just sign off for now. | 
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