Samsarans are a race of blue-skinned humanoids with transparent, water-like blood, dark hair and solid white eyes native to the Golarion setting of Pathfinder. Hailing from Tian Xia, they are an "enlightened" race who have managed to ascend to a spiritual level above that of normal humans; when they die, they inevitably reincarnate, and what's more they remember their past lives. This strong connection makes them natural sages and seekers of truth, although not necessarily good ones; their fluff kind of suggests that for the typical samsaran, their ambitions start and end with ensuring they will be reincarnated into a fresh life. Frankly, they're kind of lazy, which is a somewhat understandable side-effect of knowing that you'll get to come back and try something again and again; why hurry to complete something in this life when you'll get multiple new ones to finally finish it in?
The reason why maintaining their reincarnation is important to them is simple: a samsaran isn't guaranteed to reincarnate. If they die and are "truly imbalanced", then their soul simply ceases to be. Likewise, occasionally, a samsaran achieves "true" enlightenment and their spirit fades away from the mortal plane, going on to its ultimate fate on the planes of the dead. To preserve their race, samsarans do still breed - the vast majority of their offspring, however, are human children, which they usually foster out to human families. These children may, when they die, be reincarnated as infant samsarans, although it's unusure just who these baby blue-people are born to.
Since they have a constitution penalty, which is awful for every single class in the game, nobody plays them for any reason except to abuse one specific alternate racial trait: Mystic Past Life. This adds a number of spells from any class list of the same type (arcane/divine) to your spelllist. Just to give you an idea of how strong that is, one of the least powerful methods of abusing this is to have a Cleric who gets those juicy Paladin only spells 7 levels before a Paladin does.
Some have suggested that samsarans may have been "inspired" by the Deva of Dungeons & Dragons 4th Edition, who are likewise a race of weirdly-colored, spiritually "elevated" humanoids who possess immortality through reincarnation, but who A: are angels who gave up a divine existence to live amongst mortals, and B: came first.