fent is an adverb and stands by itself; felett (only one L - thanks vvsey!) is a postposition and stands after a noun.
A bit like the difference between yukarıda and ...-nin üzerinde in Turkish, I believe.
Similarly with lent versus ... alatt (aşağıda versus ...-nin altında).
We need to give you specific Turkish explanations, it would be much easier most of the time. :)
Btw, it is felett, not *fellett.