The term "King James Onlyism" describes a spectrum of (almost entirely) Baptist and Presbyterian Christians who believe one or more of the following things:
This last group sometimes self-identifies as "Textus Receptus Only". However, they are in reality KJV Onlyists, as the specific critical text that they use and call the "Textus Receptus" is actually just a reconstruction of the textual choices made during the KJV translation process, first created by Frederick Scrivener (1813-1891 AD).
Prior to his attempt to collate the textual choices of the King James committee, there is no evidence that the resulting underlying text existed anywhere on earth, at any point in time, in a single book. In other words, from this point of view, there was no perfect Greek text anywhere on earth until Scrivener reconstructed it around 1881, from an English translation.
The King James Version of the Bible was translated under an Anglican king, James I (1566-1625 AD). Every single member of the translation team was a confessing member of the Church of England, and additionally, almost every single one was a member of the clergy in the Church of England.
Most Baptist KJV Onlyists have never read the 39 Articles of the Church of England - which are explicitly Calvinistic, and support Paedobaptism - and are generally very ignorant of church history as a subject. The fact that their beloved translation was made exclusively by men who would consider them heretics does not appear to have slowed them down at all.
The idea of King James Onlyism is absurd on its face, as it posits that God, for some reason, waited until the 17th century to inspire a perfect Bible in a single language. And, that He did so under a Paedobaptist (not faithful to the Bible), Trinitarian (definitely not faithful to the Bible) church state, which actively persecuted and killed dissenters.
All other Christians beforehand who did not have the non-existent Hebrew and Greek originals, or who did not speak those languages? Out of luck. Every non-English-speaking person to ever live? Out of luck. Sorry! God certainly could inspire a perfect Bible in those circumstances - He did so once, in the 17th century, after all - but was evidently not interested in doing so. At least 1,500 years of chaos and confusion must pass first, for some reason.
KJV Onlyists often maintain the moat around their sect by declaring that all others have a "corrupt Bible" (at least in certain verses). The whole crux of King James Onlyism is ostensibly defending the idea of a perfect Bible. Often this is based on a deep-seated belief that in order for the Bible to be trustworthy at all, it must be perfect - otherwise, a veritable Pandora's Box is opened. However, this is especially nonsensical, as in their view, no perfect Bible existed anywhere on earth prior to the 17th century, in English.
So, if one were to ask a KJV Onlyist why they are even defending the KJV so strongly in the first place, it would usually circle back to a belief in a perfect Bible. Yet, according to this very belief, such a thing did not exist until the 17th century when the KJV was created. This position is not based on the result of any logical investigation. Instead, it is enshrining a Bible that had become very popular and was used for a long time in the English-speaking world, and so it became nostalgic and traditional in the minds of many in the 1800s, which led them to reject new translations, even when earnest scholarly work demonstrated that newer critical texts improved on the KJV's comparatively primitive critical text.
The King James Version contains the Comma Johanneum - a verse in First John chapter 5 which does not appear in the main text of extant Greek manuscripts of First John until the 14th century. It is definitely not original to "John" (himself a forger who put his own words and theology into the mouth of Jesus of Nazareth), it definitely did work its way into the Greek from Latin, and believing that it belongs in any Bible after researching its history is simply declaring that one does not care about history, evidence, or the words that their own god allegedly gave them, because of their blind allegiance to a translation made by a few dozen Anglican priests in the 1600's.
The KJV in Revelation 16:5 contains a reading which was invented by Theodore Beza (1519-1605 AD), and is unknown to any Greek manuscript tradition. It reads, "and shall be", instead of "O Holy One". This reading is absent from any Greek manuscript that has ever been discovered, and is definitely not what the author of Revelation originally wrote.
The KJV in Revelation 21:24 reads, "the nations of them which are saved shall walk in the light of it", which is actually commentary from Andreas of Caesarea (563-614 AD). This represents an addition to the much better attested reading of "the nations will walk by its light", which is how almost every other translation in every other language renders the verse.
The KJV in Revelation 22:19 reads "book of life" instead of the overwhelmingly attested Greek reading of "tree of life". The "book of life" reading originated in Latin manuscripts, and the early writers who attest to the variant wrote in Latin, not Greek. The early Greek evidence is so unanimous that this verse is often not even catalogued as containing a textual variant.
KJV Onlyism is a historically late, historically indefensible position which assumes its conclusion, and then bitterly hangs onto that conclusion even when refuted logically and historically.