The word Seraphim comes from the Hebrew word saraph (Strong's Concordance #H8314) which means, "burning ones" or a fiery serpent that is poisonous. Its reference to a class or type of angelic beings is recorded only in Isaiah 6. Other than seraphim, the KJV translates saraph as "fiery" (Numbers 21:6, Deuteronomy 8:15), "fiery serpent" (Numbers 21:8) or "fiery flying serpent" (Isaiah 14:29, 30:6).
In 740 B.C., the year Judah's King Uzziah (Azariah) dies, Isaiah sees a magnificent vision of God's throne within the temple. Above the throne he notices powerful spirit beings called Seraphim which, from the context, are quite familiar to him. After seeing these unique spirits, He briefly describes their appearance as follows.
. . . I then saw the Lord sitting upon a throne, high and lifted up, and His train filled the temple. Above it stood the seraphim; each one had six wings; with two he covered his face, and with two he covered his feet, and with two he flew. And one cried to another, and said, "Holy, holy, holy, is the Lord of hosts; the whole earth is full of His glory." (Isaiah 6:1- 3, HBFV).
Seraphim, other than possessing features similar to humans such as a face and feet, also have hands (Isaiah 6:6) and speak as humans (verse 7). Such characteristics make them like other spirit-composed servants like Cherubim which have four hands, four faces and four wings (Ezekiel 10:7 - 21).
Purpose
The primary role of Seraphim is to offer praise and worship, on a constant basis, focused on the Godhead's holiness and glory (Isaiah 6:3). Their responsibility is strikingly similar to the four beasts of Revelation who constantly give the Eternal glory, honor and thanksgiving (Revelation 4:7 - 9). Their nearness and direct access to God's throne means they are a higher order of angelic beings like the cherubim and archangels.
Why a Vision?
Why was Isaiah given a vision of God's throne with its attending Seraphim? The purpose of the vision was to commission him to serve as a prophet. The sight of God in his glory humbled Isaiah and caused him to reflect on his own sinfulness (Isaiah 6:5). After hearing his confession, one of the Seraphim angels takes a live coal from the temple's altar and touches Isaiah's lips, cleansing him of his sins (verse 7). Once forgiven, He willingly accepts God's offer to serve (verse 8).