Why do some people have the custom to wind the Tefillin-Shel-Yad towards themselves, while others wind it away from themselves? What is the reason behind these two customs, and which sects of Judaism generally follow which method?
It's pretty much a question of which eida you come from:
Ashkenazim wind inwards, Sfaradim wind outwards.
The link that @jake posted in comments illustrates this very nicely - it depends on which eida's minhag you use.
(Sorry, I don't know the reasoning behind the different minhagim - though I find it likely that each do it that way, because thats the way they do it...)