Truth himself (itself?) states,
Who am I? One name you might have for me is the world, or you might call me the universe, or perhaps God, or perhaps the Truth. I am All, and I am One. So, of course, this also means that I am you. I am the truth of your despair, the inescapable price of your boastfulness.
Truth is a being which has no physical form, and a being which regulates all alchemical exchanges that take place. He essentially exists in order to prevent humans from "playing God" with alchemy; when human transmutation is performed, for example, Truth intervenes as it is seen as an unfair (inequivalent) exchange.
In these ways, Truth is somewhat symbolic of God. It has also been compared to gods in other mythos, such as Hera, a Greek goddess who was known to vengefully challenge heroes.
Hera was known for her jealous and vengeful nature, most notably against Zeus's lovers and offspring, but also against mortals who crossed her, such as Pelias.
— Hera, Wikipedia
However, it is important to realize that Truth is not God. While it is certain that they are linked in some way, God is the entity which lies within the domain of the Gate (the eye and mass of black hands that restrain his "victims").
(The Gate, Truth sitting in front, and God within it. Source: Wikia)
When an alchemist pays the toll and is forced through the Gate by Truth (and by God), they are shown all knowledge about alchemy. Their minds can only absorb so much, but enough is clear that they are able to perform their alchemy without requiring a transmutation circle.
So, what exactly is Truth? He is a vengeful, godlike being, who is linked to God, and governs all alchemical exchanges performed by alchemists. He has no physical form, and exists only metaphysically within the mind of each alchemist.