Although seraphim are often drawn like flaming angels, they were originally six-winged beings, related to - but not a variety of - angels. Their most prominent mentions in the Bible are in Isaiah 6:
Isaiah 6:2 Above him 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.
Isaiah 6:6 Then flew one of the seraphim to me, having in his hand a burning coal which he had taken with tongs from the altar.
(These quotes were taken from the New Oxford Annotated Bible
The word in its original plural form has been translated to "six-winged angels," or, more commonly, "seraphs" (as in the New International Version).
Seraphs' antitheses, nephilim, are even less well-known than their more godly cousins. Mentioned only in Genesis, they were apparently one of the primary reasons for Noah's Flood.