They are not real. In fiction, they are like in fullmetal alchemist. Homunculus means "little human." In latin. Homunculi were believed to be real once but not completely. Though, alchemy tried to achieve human transmutation. A soul cannot be brought back because it has already reached the afterlife and cannot be brought back by any human means.
No, he doesn´t become a homunculi.
hands
Bruttunculus (e.g. homunculi/homunculus)
There is a civil war between the current homunculi Fuhrer and the forces of Edward Elric who is trying to destroy Father and the homunculi.
C. C. Krijgelmans has written: 'Homunculi'
No! He certainly did not turn himself into a homunculi. In the last episode he got transferred to the 'real world' as a result to bring Alphonse's body back from 'the gate'. Unfortunately, he still had his automail arm and leg but he did succeed in bringing Al's body back (even though Al lost his memory about his life in the armour and his body looked exactly the same as the day they tried to bring their mother back)
No he gets taken into the Gate of Truth by Truth (the white guy).
The Tattoo of Oriboris indicated that the bearer of the tattoo was not human, but a homunculus.
Alchemists are human, and since homunculi aren't really human they can't perform alchemy. Wrath on the other hand, has Ed's arm and leg - the very same ones he lost when they tried to resurrect their mother - and so he can perform alchemy.
He was defeated but Ed did not kill him. He, like Wrath were homunculi who could age.
Envy is one of the seven Homunculi that oppose Edward Elric. He (yes despite appearances Envy IS in fact male) has the power to shape-shift and possesses overwhelming physical strength (A characteristic of the sin envy; perhaps he was jealous of the other Homunculi's strengths and sought to be better than them? Or maybe he just came like that? Who knows for sure.)
The Homunculi are introduced in a variety of ways and only a few are introduced at a time (i.e. Lust, Gluttony, and Envy are introduced first and later on, Greed, and then further on, Sloth and Pride). They are revealed in the midst of chaos or lurking around and causing trouble. Try picking up the manga sometime or reading it on the Internet if you'd like to find out how they're introduced.