Well, they are different Greek terms. The word "devil" comes from the Greek word "diabolos" through the Latin "diabolus", which also gives rise to the Spanish word "diablo" and the English "diabolical". The word diabolos means "slanderer" or "accuser", and in that sense is a translation of the Hebrew shaytan, which likewise means "accuser"; the Hebrew shaytan is where we get "Satan" through its Hellenization and Latinization (both of which are "Satan").
Belial is the Hellenization of beli-yaal, meaning "without value" or "worthless", which during the Second Temple period came to be used in reference to the chief of the fallen angels, referred to in Second Temple literature often as Samael or Satan. "Samael" is the closest there is to an actual name given for the conceptual chief of the fallen angels, "Satan" is chiefly a title, as would be "the devil" which is effectively a translation; though Belial is treated as a name for this creature in the literature.
From one of the Dead Sea Scrolls, specifically the War of the Sons of Light Against the Sons of Darkness, we find this passage:
"
You made Belial for the pit, angel of enmity; in darkness is his domain, his counsel is to bring about wickedness and guilt. All the spirits of his lot are angels of destruction, they walk in the laws of darkness; towards it goes their only desire." -
Belial - Wikipedia
The identification of the serpent with Satan likewise comes to us from the Second Temple period, the most affirmative I am aware of is in the work known as the Life of Adam and Eve, in which the serpent is explicitly described as being the devil; this tradition is also nodded to several times in the New Testament, perhaps most notably in the Apocalypse of St. John where the devil is frequently depicted as "the serpent" and "the dragon"; St. Paul's statement that "For the God of peace will soon crush Satan underneath your feet" is a possible allusion to the Eden prophecy concerning the child of Eve crushing the serpent, long understood in Christianity to be a prophecy about Christ conquering the devil.
So, in a word, the most clear position that exists within the biblical material is that there are fallen angels, known as "devils" and "demons" in the Greek text, and the chief of these is known variously as "Satan", "the Devil" and more rarely "Belial"; which seem to largely be taken from Second Temple Jewish ideas about fallen angels and their chief "Belial" or "Samael", identified as "the Satan"; such views did not survive in mainstream Judaism after the destruction of the Temple, but continued in Christian thought.
-CryptoLutheran