1) "Idioms are peculiar. Even if one knows all the words that make one up, the meaning must be learned. That is, understanding the meanings of "get" and "along" individually does not really help in deciphering that "get along" can mean "to be friends with". Metaphors, on the other hand, create images in a reader's mind. If you know all the words that make up a metaphor, you have a good chance of getting the intended image. "Joe and Sue are bread and butter together" is a metaphor for Joe and Sue's good relationship. (I just pulled that one out of my head; I know it sounds terrible.) So, if you know all the words (bread, butter, together, etc) and you come from a culture where people enjoy eating bread with butter, you can easily undertand the meaning of the metaphor. More examples: "I had my lunch." (Idiom. It means "I ate my lunch", rather than the literal "I possessed my lunch".) "I wolfed my lunch." (Metaphor. I acted like a wolf with my lunch; I ate it like a wolf would.)" Source and further information: http://www.englishforums.com/English/IdiomsVsMetaphors/cqwl/post.htm 2) "An idiom is a term or phrase whose meaning cannot be deduced from the literal definitions and the arrangement of its parts, but refers instead to a figurative meaning that is known only through common use. In linguistics, idioms are widely assumed to be figures of speech that contradict the principle of compositionality; however, this has shown to be a subject of debate. It may be better to refer to idioms as John Saeed does: words collocated together happen to become fossilized, becoming fixed over time." "An idiom is generally a colloquial metaphor — a term which requires some foundational knowledge, information, or experience, to use only within a culture where parties must have common reference. Idioms are therefore not considered a part of the language, but rather a part of the culture. As cultures are typically localized, idioms are more often not useful for outside of that local context. However some idioms can be more universally used than others, and they can be easily translated, metaphorical meaning can be more easily deduced. The most common idioms can have deep roots, date back many centuries, and be traceable across many languages. Many have translations in other languages, and tend to become international. While many idioms are clearly based in conceptual metaphors such as "time as a substance", "time as a path", "love as war" or "up is more", the idioms themselves are often not particularly essential, even when the metaphors themselves are. For example, "spend time", "battle of the sexes", and "back in the day" are idiomatic and based in essential metaphors. These "deep metaphors" and their relationship to human cognition are discussed by George Lakoff and Mark Johnson in their 1980 book Metaphors We Live By. In forms like "profits are up", the metaphor is carried by "up" itself. The phrase "profits are up" is not itself an idiom. Practically anything measurable can be used in place of "profits": "crime is up", "satisfaction is up", "complaints are up" etc. Truly essential idioms generally involve prepositions, for example "out of" or "turn into"." Source and further information: http://en.wikipedia.org/wiki/Idiom 3) "Metaphor is language we use to compare things, but without using "like" or "as" - because that would be a simile. Another sort of metaphor is a "conceptual metaphor". A metaphor very often uses the verb "to be": "love is war", for example, not that a writer sees "love as war" (this is a simile). Poetry includes much metaphor, usually more than prose. Idioms use metaphors, or are metaphors: for example, the English phrase to kick the bucket means to die." Source and further information: http://simple.wikipedia.org/wiki/Metaphor Further information: http://en.wikipedia.org/wiki/Metaphor