事(こと)なの(A) is (Description of A), (A) means that (Description of A)

ことなの combines こと with the explanatory ～んです／のです form and is used to give or ask for background with an emphatic, explanatory tone. ということ also turns a phrase into a noun, meaning 'the fact that' or 'the act of (A)', but is typically used to clarify something without the added emphasis of ん／の.

These related expressions both use the noun こと, or 事 in kanji. On its own, こと turns a verb or adjective into a noun phrase, e.g. 食べる 'to eat' becomes 食べること 'the act of eating'. When combined with the explanatory/emphatic なの, it conveys meaning or definition, e.g. 'It means that (A)' or '(A) is how it is.'

Both use こと to summarize information, but with different nuances. ことなの means 'it's that (A)' and is often used to clarify or define something, as in '(A) means (B)' or 'that means (A).' ということだ can also be used in this way, but additionally functions to report hearsay, e.g. 'I hear / They say (A).' ことなの, without the quoting function of という, is not used to report what others have said.

ことなの and というのは can both be used in explanations, but differ in position and role. ことなの appears at the end of a sentence to emphasize a conclusion or explanation. Literally, 'it's that it's (A)'. というのは follows a noun or phrase to introduce it as a topic to be explained. They can be used together, e.g. 人生というのはこういうことなんだ ('This thing we call life... this is what it means.')

These two expressions are similar in that both can be used when offering or seeking explanations. ことなの is used at the end of a phrase or sentence, often to clarify a particular point, e.g. 今日は来ないということなの？ ('So does that mean he's not coming today?'), while ということは begins a sentence and sets up a conclusion based on what was just said, e.g. ということは、今日は来ないんだ ('So that means he's not coming today').