The shortest answer in both the books and TV series is "we don't know yet".
We do not know yet what the origin of the Others is. Some characters speculate that they appear in the coldest winters, and they seem to be related to ice and cold (probably the "Ice" part of the "Song of Ice and Fire" theme). They might or might not be related to the resurgence of magic in the world, but we at least know they predate the birth of the dragons, because we're shown an encounter with the Others in the prologue, before Daenerys' eggs hatch. I mention this because, in turn, some characters (e.g. the warlocks of Qarth) believe the return of the dragons brought magic to the world, but since we know the Others predate Dany's dragons, they must have been brought by a different kind of magic, if any.