というのは

というのは takes a word or an idea as the topic so you can say what it means or what it is like: "X means", "what is called X is". It follows a noun or a quoted term directly. When the sentence defines the word, it usually ends in ことだ or といういみ意味だ.

というのは or は

A plain は talks about the thing itself. というのは steps back and talks about the word or the idea: what it means, or what it is like in general. That's why it suits definitions and general truths.

After a whole clause, というのは turns a claim into the topic: "the idea that X", "the story that X".

Giving a reason

At the start of a sentence, というのは introduces the reason for what you just said: "the thing is", "that's because". The sentence it starts ends in からだ or のだ.

In casual speech というのは becomes っていうのは or just って.