The only reference in Scripture to the seraphim is in the book of Isaiah (though the apocryphal Book of Enoch makes several references to the seraphim as well):
Above him [God] stood the seraphim. Each had six wings: with two he covered his face, and with two he covered his feet, and with two he flew. And one called to another and said: "Holy, holy, holy is the LORD of hosts; the whole earth is full of his glory!" And the foundations of the thresholds shook at the voice of him who called, and the house was filled with smoke. And I said: "Woe is me! For I am lost; for I am a man of unclean lips, and I dwell in the midst of a people of unclean lips; for my eyes have seen the King, the LORD of hosts!" Then one of the seraphim flew to me, having in his hand a burning coal that he had taken with tongs from the altar. And he touched my mouth and said: "Behold, this has touched your lips; your guilt is taken away, and your sin atoned for."
(Isaiah 6:2-7 ESV)
According to the translation notes of the NET Bible:
[The] Hebrew שָׂרָף (saraf, "seraph") literally means "burning one," perhaps suggesting that these creatures had a fiery appearance (cf. TEV, CEV "flaming creatures"; NCV "heavenly creatures of fire"). Elsewhere in the OT the word "seraph" refers to poisonous snakes (Num_21:6; Deu_8:15; Isa_14:29; Isa_30:6). Perhaps they were called "burning ones" because of their appearance or the effect of their venomous bites, which would cause a victim to burn up with fever. It is possible that the seraphs seen by Isaiah were at least partially serpentine in appearance.
There may be indirect references to the seraphim in other passages, such as Psalm 104:4 (Hebrews 1:7): "He makes His messengers winds; His ministers a flaming fire." It has been speculated that Satan was a seraph because he has been described as a serpent.