月とスッポン (tsuki to suppon) (also “月と鼈” or “月とすっぽん”) literally means “the moon and a (Chinese) soft-shell turtle”. Figuratively, it describes two things that are utterly different from each other. The closest English translation might be “(be different like) chalk and cheese” or “apples and oranges”, but unlike these expressions, the Japanese idiom usually suggests that one is much better than the other.
This idiom is often used when you compare two things/people and emphasise that the one is undoubtedly superior to the other. For instance, you may compare the governments of two countries and say they are 月とすっぽん, highlighting how good/bad one of the governments is compared to the other.
It is often said that this idiom comes from the fact that the moon and (the shell of) a turtle are similar in terms of shape (i.e. both are round) but very different in terms of beauty (i.e. the moon is much more beautiful than the turtle).
スッポン (Suppon) in Japan
In Japan, it is widely believed that スッポン (a Chinese soft-shell turtle) is – although not commonly served – one of the most effective food ingredients for fueling (men’s) sexual energy and stamina, as well as for one’s health and beauty. Famous Japanese foods using スッポン include “Suppon hot pot”, “Suppon soup”, and even “Suppon blood mixed with wine/sake”, which you may want to try in Japan to get your blood pumping.
His acting and hers were exactly “the moon and suppon” (i.e. His acting was way much better than hers)
五十歩百歩 (gojuppo hyappo)