Fate and Destiny are the creators of worlds. They are a couple, but one bound by no formal bonds other than that of love. The two of them are the corporal agents of the higher Powers - Time - the chief Power, Season - the second, and then the lesser but still important ones: Love, Hate, Desire, Rage, Pride, Sloth, Hunger, Despair, Fear and Grief. After these Powers, we have the most inferior ones: Wind, Rain, Snow, Sun, Moon and Sea.
Fate controls the lives of all except the Fatechildren - or the Tiidillers, in Tan'ganta - who have been chosen by one of the Powers for a particular purpose, though the Fatechild very rarely finds out which one because not even Fate herself knows.
Fate controls the Fatethreads, which weave together in an eternal tapestry that will never end. The Threads wind together and cross paths to show where people meet; they end to show when someone dies; and they begin to show where someone is born.
The Fatechildren often have very short Threads, but that isn't to mean that they are going to die, simply that Fate's control ends there. However, this thread grows longer for however long the Child lives, and weaves its own way through the pattern.
A long time ago, Fatechildren were hunted because they were thought of as possessed by demons. Those that hunted them didn't understand that the Children were specially chosen by the Powers that ruled all the worlds. Once the Powers realised what was happening, they stopped choosing people for whatever purposes they had been choosing them for. Now, it only ever happens every few millennia.
There aren't many who can read the Fatethreads, and those who can were taught by Fate. She tends to choose those close to her or her relatives, for example, all of the Elders know how the read the Threads, but very few of their children. Daniel - son of Lexica - was never taught because Fate knew that he was going to be exiled and had never trusted him enough to begin with. Frae - Daniel's brother - was never taught because Fate knew him to be treacherous and untrustworthy. Lucius was taught because he is pure-hearted and loyal.
Destiny is the one who controls where you end up at the end of your life. He sees to what people think about you after you're gone, and what happens as a result of your death. While his part is very small, it is very important.
Several thousand years ago, Destiny performed experiments on unsuspecting victims. He would perform a transfusion, where his blood would enter the veins of the other person. His blood would then disguise itself as the host's blood, meaning it was undetectable. While this made no direct effect on the hosts' lives, upon their death, it brought them back to life as horrible monsters who knew no self control and had no memory of their loved ones or even their previous lives. They were hunted down and, originally, killed--but this only worsened the situation, seeing as that was how they'd become monsters to begin with. After this was realised, they were trapped - though many innocents were killed in the attempt of this - and then locked in the deepest caves, or the darkest pits, at the bottom of the deepest oceans. Since then, Destiny hasn't performed another of these experiments. Well, until Robert, that is.