In Break it up, you two!: The zero width non-joiner I discussed the purpose of the zero width non-joiner, which is to request that two adjacent characters be rendered without a ligature. Conversely, the zero width joiner requests that they be rendered with a ligature. Of course, it is up to the rendering engine to make the final decision as to how the rendering is done, but at least you can make your intentions clear.
Another character that seems very similar to the zero width non-joiner is the zero width space. Both of them have no width, and both of them break up ligatures. So what’s the difference?
Well, one of them is a space, and the other one isn’t.
The zero width space is used to indicate where one word ends and another word begins, even though there should not be any space rendered at the word boundary. This is significant for languages which have the concept of words, but not of spaces. For example, the Thai and Korean languages use multiple characters to represent words, but traditionally do not insert spaces between words. The words just run together, and readers are expected to use their experience with the language to know where one word ends and the next begins.