A word in English is relatively simple to spot: words are separated by whitespace or (some) punctuation. Even in English, though, there can be controversy: is you’re one word or two? What about o’clock, cooperate, half-baked, or eyewitness?
Languages like German or Dutch combine individual words to create longer
compound words like Weißkopfseeadler (white-headed sea eagle), but in order
to be able to return Weißkopfseeadler as a result for the query Adler
(eagle), we need to understand how to break up compound words into their
constituent parts.
Asian languages are even more complex: some have no whitespace between words, sentences, or even paragraphs. Some words can be represented by a single character, but the same single character, when placed next to other characters, can form just one part of a longer word with a quite different meaning.
It should be obvious that there is no silver-bullet analyzer that will miraculously deal with all human languages. Elasticsearch ships with dedicated analyzers for many languages, and more language-specific analyzers are available as plug-ins.
However, not all languages have dedicated analyzers, and sometimes you won’t even be sure which language(s) you are dealing with. For these situations, we need good standard tools that do a reasonable job regardless of language.