と言(い)う事(こと)~ing, The ~ that ~ (Nominalization)

ということ turns phrases into nouns and often clarifies what was said, e.g. 日本に行くということ 'the fact that he's going to Japan'. Adding だ forms ということだ, which is used to report information second-hand, e.g. 来年から海外に住むということだ ('I hear he'll be living abroad from next year.').

ということ turns phrases into nouns and is often used to clarify what was said. When followed by は as ということは, it marks the phrase as a topic for further explanation or comment. ということは can also begin a sentence to restate or interpret what came before, like 'So what you're saying is (A)' in English.

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.

ということ turns phrases into nouns and is often used to clarify or restate something, e.g. 日本に行くということ 'the fact that (he) is going to Japan'. という by itself links a phrase or name to a noun, as in ポチという犬 'a dog named Pochi' or 来年日本に行くという学生 'a student who is going to Japan'.

ということ and というのは both turn phrases into nouns. ということ is often used to clarify what was said, while というのは introduces a topic for further explanation or comment. というのは can also start a sentence to mean 'what I mean is (A)' or 'that is to say (A)'.

こと attaches to the dictionary form of verbs to turn them into nouns, e.g. 行くこと '(the act of) going'. ということ has a similar function, but adds the quoting phrase という, so it's often used to clarify information or reported speech, e.g. 日本に行くということ 'the fact that (he) is going to Japan'.