It is said that whoever lays eyes on a Seraph, he would instantly be incinerated due to the immense brightness of the Seraph. They are described as very tall, with six wings and four heads, one for of the cardinal directions. One pair of wings are for flying, one for covering their eyes (for even they may not look directly at God), and one for covering their feet (which is almost certainly a euphemism for genitalia). They are in the direct presence of God.
In Isaiah's call-vision in the Temple, he sees Seraphim surrounding the throne of God, singing praise to God; the "Thrice Holy" hymn (ch 6). In this instance they are angelic beings but in the Book of Numbers, seraph-snakes are sent to punish the Israelites.