The short answer
Seraphim appear by name in one biblical vision, Isaiah 6. Each has six wings, they call "Holy, holy, holy," and one presses a live coal from the altar to Isaiah's lips.
The seraphim appear by that name in only one biblical vision, Isaiah 6, set "in the year that king Uzziah died." Isaiah sees God enthroned in the Temple. "Above it stood the seraphims: each one had six wings; with twain he covered his face, and with twain he covered his feet, and with twain he did fly." They call to each other, "Holy, holy, holy, is the LORD of hosts," and the doorposts shake while the house fills with smoke.
Isaiah fears he is doomed, being "a man of unclean lips." Then "flew one of the seraphims unto me, having a live coal in his hand, which he had taken with the tongs from off the altar: And he laid it upon my mouth." His guilt is removed, and he volunteers for his mission: "Here am I; send me."
The word seraph comes from a root meaning "to burn." Elsewhere in the Bible it names creatures that are not angels at all. In Numbers 21 the "fiery serpents" that bite the Israelites are seraphim, and Isaiah mentions a "fiery flying serpent." Many scholars link the image to the Egyptian uraeus, the rearing cobra with spread hood, sometimes shown with wings, which appears on seals found in Judah. Later Christian writers ranked the seraphim as the highest order of angels, closest to God. Their song became a central part of Jewish and Christian worship.