A classification system is a system for arranging data into groups in a useful way. With classification systems, items are classified according to some criteria that is useful for the purpose intended. However, different systems can be used for the same data, because different systems have different purposes.
A few well-known classification systems are those for classifying library books, astronomical objects, and living things.
Dewey Decimal System
The Dewey Decimal System is a classification system for books in a library. It was invented by Melvil Dewey in 1876, and divides books into ten categories, each of which are further subdivided into ten sub-categories, each of which is again subdivided into ten sub-sub-categories, although not all of these are used. Some of the unused ones were previously used, as the system has been modified over time. One change is that the system was originally designed to cater for all books, but is no longer much used for fiction.
The Dewey system is not the only system used, however. A popular alternative is the system used by the United States' Library of Congress, and other systems include the BISAC system used mainly in bookstores but also some smaller libraries, and the NLM classification system often used in medical libraries. What this illustrates is that there is more than one way of classifying objects, and different systems can be used in different circumstances.
Planets and other extraterrestrial objects
The ancient Greeks considered any point of light in the sky to be a "star", but noticed that while most stars moved across the night sky in concert, some of them moved in a more complicated pattern. These stars were dubbed asters planetai which literally means "wandering stars", and it from this term that we get the word "planet". The Ancient Greeks also believed that the Earth was the centre of the universe, with the sun, moon, and stars (including planets) orbiting the Earth.
Later, and following the discovery by Christian scientists that the Earth and planets orbited the sun, the definition of "planet" was altered to refer to bodies orbiting the sun, and a star was a hot ball of gas. This meant that our sun was classified as a star.
Then in 1801 the first of several small "planets" were discovered in solar orbit between Mars and Jupiter. By the middle of the century, with 15 of these small bodies found, astronomers started calling them asteroids, keeping "planet" for the larger bodies. Pluto was discovered in 1930, and it was classified as a planet, making it the ninth known planet in the solar system, but it was significantly smaller than any of the outer planets, and improved knowledge about its size eventually brought it down so much that it is now believed to be significantly smaller than Mercury.
In the 1990s and the following decade, further small planets were discovered beyond the orbit of Pluto, including one found in 2005 that was larger than Pluto. These discoveries led the International Astronomical Union in 2006 to change the classification system for planets, introduce a new classification of "dwarf planet", and reclassify Pluto and similar bodies into that category. Nothing about Pluto changed, except for how it was classified, as the classification system was modified.
Linnaean taxonomic system
The Linnaean taxonomic system was developed by the creationist Carolus Linnaeus in about 1735, one and a half centuries before Darwin popularised the theory of evolution. Some ideas introduced by Linnaeus, especially binomial nomenclature, giving each species a unique identification by combining a genus name with a species name, are still used by biologists today. Since Darwin and his popularisation of the idea of common descent, the system has, like many other classification systems, been modified to take the presumed evolutionary relationships among extinct as well as extant species into account.
Partly because the Bible was written before modern scientific classifications came into vogue, and partly because it has different goals than science, the Bible sometimes uses classification systems that are unfamiliar to modern minds and this has led to atheists and other critics of the Bible claiming that the Bible got some details wrong.
The classic example is of the Bible classifying bats as birds. Critics charge that the Bible got biology wrong, as under the Linnaean system, bats are mammals, not birds. However, the Bible (which was in any case written well before the Linnaean system was developed) was simply using a different classification system, where Hebrew word for "bird" referred to creatures with wings.
A less-well-known example that many Christians get wrong is the definition of "life". The Bible indicates that there was no death before Adam sinned, but critics will sometimes point to the death of plants or bacteria that would have died before this. However, this, again, relies on a modern definition of what is alive, whereas the Bible makes clear that it is only talking about nephesh (Hebrew נפש) life, referring to things that breathe or have emotions, feelings, and consciousness. As the Bible is using a different classification system that that used by modern biologists, it is not posing an impossible situation in claiming that originally there was no death.