As a noun:
* "Fun" refers to the enjoyment or pleasure that something gives. For example, "We had a lot of fun at the party."
As an adjective:
* "Fun" describes something that is enjoyable or pleasurable. For example, "It was a fun game to play."
In the phrase "word fun," it's a bit ambiguous. It could be:
* A noun phrase: Referring to the noun "fun" and some aspect of it as a word. For example, "The word 'fun' is a simple and happy word."
* A phrase describing something else: Meaning that something is enjoyable and is described by the word "fun." For example, "That was word fun, wasn't it?"
Without context, it's hard to tell which meaning is intended.