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I have seen both and am uncertain what difference there is. As far as I understand, both mean What's that? or What is it?

What is the difference? Is either preferable, or is one of them slang?

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up vote 3 down vote accepted

Qu'est-ce que c'est? is your regular What's that?, not very formal, but not specially familiar neither.

Qu'est-ce que c'est que ça? is less formal, and bears a sense either of negativity, anger, or surprise. It could be translated into What's that thing?.

On the other side of the spectrum, you could use the very formal Qu'est-ce? or Qu'est-ce donc?.

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If I said something crazy like Quelle est cette chose? would that be blatantly weird? – Aerovistae Dec 2 '13 at 9:34
Depending on the context, it might be plain weird, or too formal. I honestly don't see a context where this expression would be chosen over the others, at least orally. – Alexis Pigeon Dec 2 '13 at 9:40
Worst case scenario, one would say “Qu’est-ce [que c’est que] cette chose ?”. If you want to go less formal, I would say “C’est quoi ?” for “What’s that?” and “C’est quoi ce truc ?” for “What’s that thing?”. – Édouard Dec 2 '13 at 10:15
Wouldn't you say "Qu'est-ce que cette chose?" rather than "Qu'est-ce cette chose?" ? The latter seems pretty unnatural to pronounce... – Alexis Pigeon Dec 2 '13 at 10:29

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