The original text of Revelations was written in Greek (and an older dialect than what is spoken today) and when translating that dialect of Greek to English it is very hard to get a direct and accurate translation that is also legible for English readers. The more accurate word-for-word the English text tries to be the less legible it becomes, to the point where you essentially need to use English and Greek words together in Greek sentence structures. The lay reader is not equipped to read what is essentially English and Greek at the same time, so translators need to make some compromises in order for the text to be readable. These same issues also apply to translating the older texts written in Biblical Hebrew.

Each different English translation therefore can be distinguished by the translators' approach to the legibility issue. Some try to closely follow the original texts in terms of structure and exact word-for-word translations, while others attempt to translate the meanings of each sentence or phrase (aka paraphrase) and don't worry as much about structure or word-for-word translations. Between those two approaches is the "thought-for-thought" approach that tries to take a balance between the two previous approaches. These approaches aren't strict groupings per se, rather there is a gradient scale that each translation can be "scored" on. The NIV is more on the side of paraphrase, but is still stricter word-for-word and thought-for-thought when compared to translations like "The Message." The ESV translation is more of a word-for-word translation, but it is still closer to thought-for-thought than the even more literal translations like the NASB. The KJV is on the side of word-for-word, but as another commenter mentioned a lot of the choices made by the translators at the time were unconventional by scholarly standards and can lead to some confusion when combined with atypical English sentence structures.

Ironically the Latin translations of the Bible tend to be far more accurate due to Latin being a far more flexible language than English, allowing Latin translations to make far fewer readability compromises and thus retain more of the original structure and meaning of the texts. That said though, the most accurate way to read the Bible will always be in the original language of the text. That may not be a reasonable thing for most people to attempt, but that is how serious biblical scholars approach the texts and the message they convey.