The word Seraphim (one seraph, two or more seraphim) means “burning ones” or nobles. They are also sometimes called the 'ones of love' because their name might come from the Hebrew root for 'love'. Seraphim are only fully described in the Bible on one occasion. This is in the book of the prophet Isaiah, when he is being commissioned by God to be a prophet and he has a vision of heaven 1.
So these types of heavenly beings have six wings, but they only use two of them for flying. It sounds strange to use wings to cover your face and feet. They may well cover their face because, being so close to God, they would witness His full glory which would be too powerful to behold. Feet are considered 'unclean' and so not worthy to be shown to God. (Some scholars also think that 'feet' could actually mean 'genitals'.) We're not told how many Seraphim there are, but it's more than one.
Their position is flying above God's throne, unlike the Cherubim who are beside/around it. Their primary duty is to constantly glorify and praise God and they may also be the personal 'attendant' angels of God. Their eternal song "Holy, holy, holy is the LORD Almighty; the whole earth is full of his glory." has been used by Jews and Christians for thousands of years to join with the angels in praising God. In Hebrew, to use the same word three times to describe something means that the person/object is utterly like the word. So calling God Holy three times means that God is utterly and perfectly holy.
In Jewish folklore, and some later Christian works, the Seraphim are said to be the highest rank of angel. This is probably because of their very close proximity to God.
In art, Seraphim are often red (because of their names 'burning ones') and are shown holding a flaming sword with the words 'holy, holy, holy' on the blade.
The coal in Isaiah's vision, that touches his lips, is used to signify that Isaiah is now purified and fit to be a prophet. The coal came from the altar in heaven, so would have been very powerful. Fire is also used in many religions and faiths as a way of purifying and cleansing something.
