The word appears in the Bible in Isaiah's vision recounted in Isa 6 (vv2 & 6) where they describe angelic creatures. Seraphs is found in NIV & NRS, while seraphim is used in NKJ (KJV's 'seraphims' incorrectly doubles the plural).
Isaiah describes them as having six wings, and seemingly leading in the heavenly worship, extolling repeatedly the holiness of God, their voices so powerful "the doorposts and thresholds shook and the temple was filled with smoke"(v4).
Whatever these creatures may be—and many commentators have associated them with the cherubim—they were no mere indifferent agents but moral beings, empowered to execute an atoning cleansing responsibility (v6).
As the word—which is plural in the Hebrew—is only used here to describe such creatures, the origin of the word remains a mystery. The Hebrew word seraph (singular) appears elsewhere in the Old Testament, where they refer to the venomous snakes that killed many of the Israelites in the wilderness (Num 21:6, 8; Deut 8:15) and the "serpents" (NIV) or "fiery flying serpent" (NRS, NKJ) threatened upon the Philistines (Isa 14:29) and the Negev (Isa 30:6); the differences in their referents suggest the plural and the singular forms of the word represents two entirely different categories of things. (Just as a matter of clarification: snakes are venomous, if what is meant is that their bites kill; they are poisonous (which is the adjective used in the NRS at Num 21:6) only if they kill when their flesh is consumed.)
The Hebrew verb saraph is quite common and always refers to actual burning; consuming or destructive burning, not those used in cooking or in metaphors. This usage, however, sheds no light on the origin of how the nouns are used for venomous serpents (in the case of the singular) or for the angelic creatures (in the plural).
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