Translation:What is the difference between the machines?
If I had gotten this translation from English to Czech, I probably would have written "co." But now, seeing that it's actually "jaký" that is used, I'm reminded of simple sentences earlier in the course such as "Jaká je Kateřina" and "Jaký je František (What is K or F like?). So I'll probably fix this construction in my head as "what is the difference (like) between X and Y." Hopefully, it will work...
Possibly, I am not sure. See https://blog.oxforddictionaries.com/2015/06/29/grammar-myths-among-or-between/