I'm a U.S. native speaker. Yes, it is frequently used. It means "a quick, condensed summary." It means "in the fewest possible words, but perhaps not complete or completely accurate." "In a nutshell, the way the U.S. electoral college system works is that it is the states, not the people, who vote." "In a nutshell, English words come from two main sources, Anglo-Saxon and Latin." I think the origin of the usage is a famous quotation from Shakespeare's play, "Hamlet:" "O, God, I could be bounded in a nutshell and count myself a king of infinite space, were it not that I have bad dreams." It doesn't really make sense because Hamlet was not using the phrase "in a nutshell" to mean "in few words," but nevertheless I think that's how the phrase "in a nutshell" came to become a common idiom. ADDED: Well, an Internet search suggests that I'm half-right. It probably does come from Shakespeare, but Shakespeare himself may have gotten it from a classical source, perhaps Pliny the Elder. It was a reference to a handwritten version of "The Iliad" so small it could fit into a nutshell.

Just to add to what Dan and Scott have said: Yes, it's used in all English-speaking countries. It's a very common idiom. Yes, it's common in both spoken and written English. Many of the idioms which students learn are rarely used by native speakers, but 'in a nutshell' is frequently used, especially in speech. The reason it is so common is that it's very useful as a 'filler'. It means 'basically' or 'in a few words' in summary' 'in brief' or 'in essence'. We use this is conversation to buy ourselves a few seconds while we think of what to say next. 'So, why do you think the project failed?' 'Well...in a nutshell....' This tells the listener that you are going to summarise the main point, but it gives you a little time to think about how to phrase the rest of the answer. As for 'That's strange. In Spanish that sentence doesn't make any sense!' - well, that's exactly what an idiom is - in a nutshell (!). This is the dictionary definition of an 'idiom' : "a group of words established by usage as having a meaning not deducible from those of the individual words." An idiom is an expression which you can't take literally. Here's a list of Spanish idioms that don't make any sense in English - http://www.fluentu.com/spanish/blog/spanish-idioms/. The individual words don't necessarily 'make sense', but the whole meaning does, and (in most cases) you can't translate the idiom directly from one language to another. I hope that helps.

Yes. I live in the United States. It means, "here's a brief summary"