An idiom is a phrase or form of words which is used in a particular way in one language and may not translate into another. For example in British English the idiom ' to kick the bucket' means to die, but would probably not mean this if you put it into another language. A proverb is a saying, usually old, which is supposed to contain wisdom or useful information: 'A stitch in time saves nine' is an example of a proverb.