Seraphim are an integral part of the Orthodox Judaic angelic hierarchy, where they represent the fifth rank out of ten ranks of angels. Within the Hebrew Bible or Tanakh itself, there is only one mention of the Seraphim, and it appears in Isaiah Chapter 6, verses 1 through 3. It's here that Isaiah describes the prophet's vision, and the world is first introduced to Seraphim angels. The verses state, "In the year that King Uzziah died, I saw the Lord seated on a throne, high and exalted, and the train of his robe filled the temple. Above him were Seraphs, each with six wings, with two wings they covered their faces, with two they covered their feet, and with two they were flying. And they were calling to one another, holy, holy, holy is the Lord Almighty, the whole earth is full of his glory". From this sole yet powerful passage, the Judaic angelic legacy of the Seraphim was born, and eventually passed on to Maimonides, who went on to further define the Seraphim in depth within his exposition of the Judaic Angelic Hierarchy.
In Isaiah Chapter 6, verses 1 through 3, the prophet recollects his encounter with the Seraphim in detail. Although he doesn't specify how many Seraphim he actually saw, we can obviously assume that there were more than one of these heavenly beings surrounding God's throne. Describing the initial encounter, Isaiah goes on to state that the "foundations of the thresholds" of the Temple were shaken by the sheer power of the Seraphim song. This is the same Trisagion song which is referred to in both Judaic and Christian prayer.
As the prophet's story continues, a solitary Seraph removes a burning amber coal from the holy alter, then approaches Isaiah and presses it firmly against his lips. This holy gesture which echoes New Testament scripture, is said to have purified Isaiah of all his worldly sin. Isaiah then goes on to describe the Seraphim as "living beings", with feet, hands, and faces, although one should not automatically assume from that initial description that the Seraphim do indeed have a humanlike form. Curiously, the way the prophet speaks of the Seraphim within these verses, is as though his intended readers are already familiar with them, possibly through unknown oral traditions.
Using the aforementioned scripture as a foundation, Maimonides goes on to answer many unknown questions regarding the Seraphim. In his Yad ha-Chazakah, Yesodei ha-Torah, he ranks the Seraphim as the fifth highest form of celestial being. With regard to their aesthetic attributes, he goes well beyond all initial conceptions of the Seraphim, and describes them as having actual humanlike qualities. It's this literary cornerstone in addition to New Testament scripture, that heavily influences our contemporary interpretation of what the prophet Isaiah actually encountered that day in his vision.
The word Seraphim which originates from Hebrew, is the plural version of the word Seraph. So obviously when you say Seraphim, you are referring to more than one Seraph angel. The word Seraph itself seems to originate from either the Hebrew verb saraph, which means "to burn", or the Hebrew noun saraph, which translates to mean "fiery flying serpent". There are still some academic disagreements over whether or not the word Seraph originated in Hebrew, as some scholars have concluded it may have much older roots dating back to ancient Egyptian, or possibly even Babylonian origin.
Outside of the initial reference to the Seraphim in the Book of Isaiah Chapter 6, there are several more occurrences of the word saraph that are not used in direct reference to angels or celestial beings. Due to the fact that these occurrences of the word are used in varying contextual settings both literally and metaphorically, there are still some conflicting views on what type of angels were actually being described in those first introductory verses. The consensus is obviously split between the noun and verb saraph, with some scholars having concluded that the Seraphim are "fiery winged serpents", while others only attribute the verb to them, meaning they were not serpents, but merely "fiery beings" radiating intense light.
The first non angel related occurrence of the word saraph, is in the Book of Numbers Chapter 21, verse 8, which has a passage that reads "and the Lord said to Moses, make yourself a fiery serpent, and set it upon a pole, and it shall come to pass that every one that is bitten, who looks upon it, shall live". This verse is translated by some to reference Moses' construction of a brass fiery colored serpent, which was meant to symbolize God's wrath and judgement over the wandering Israelites. It's thought it may have been a tool to counter any descent amongst the vast number of Moses' people, and to act as a sort of moral cattle prod during their journey through the Sinai.
The second non angel related occurrence of the word saraph, is in Deuteronomy Chapter 8, verse 15. The verse reads "who led you through that great and terrible wilderness, amongst fiery serpents, scorpions, and drought, where there was no water, who brought forth water for you out of the rock of flint". Most scholars have concluded that this occurrence of the word saraph was a literal reference to the fierce and dangerous reptiles that inhabited the Sinai during that time. The region today is still inhabited by several extremely venomous snakes, such as the Horned Viper, Burton's Carpet Viper, and the Black Cobra, all of which would have been absolutely deadly without modern medical assistance.
The third non angel related occurrence of the word saraph can be found in Isaiah Chapter 14, verse 29. The verse states "all you Philistines, do not rejoice that the rod of the one who struck you is broken, because a viper will come from that snake's root, and his descendant will be a flying, fiery serpent". Obviously the occurrence of the word saraph here is non literal, and is simply used to describe the coming wrath of the descendants of the Israelites upon the Philistines. When analyzing the use of the word in this setting, one could make a strong argument that when the Seraphim were first described in Isaiah Chapter 6, the author did indeed mean fiery flying serpents, and fierce protectors of God's throne.
The fourth and final non angel related occurrence of the word saraph appears in Isaiah Chapter 30, verse 6. The verse reads "This is the divine revelation about the animals in Negev. My people travel through lands where they experience distress and hardship. Lions and lionesses live there. Vipers and poisonous snakes live there. They carry their riches on the backs of young donkeys and their treasures on the humps of camels to a nation that can't help them". Here we have another literal reference to the poisonous snakes and vipers that inhabit the Sinai. The threat these snakes must have posed to travelers at that time, was likely forefront on the minds of these people to such a degree, that it ended up in Biblical scripture more than once.
After reviewing all of the occurrences of the word saraph, and simply asking myself the question, were Seraphim originally described as fiery winged serpents, or merely burning beings of light, I tend to lean toward burning beings of light. What drives me to that conclusion is that God himself is described as burning with light, or simply too bright to look upon with human eyes. Passing that attribute along to celestial beings that directly surround his throne, seems more consistent with Old Testament scripture than burning winged serpents. Also notice how they were described as having feet, something serpents do not have. Obviously I'm just barely scraping the surface on an extremely complex debate amongst scholars. Though even with that said, there will likely never be a definitive answer, so it's obviously up to you to come to your own conclusion.