Idioms can be a single word, a phrase or clause, or they can be a complete sentence or expression. People often explain their use as figures of speech. Many are indeed figurative, sometimes simply visually exaggerated. For example, it must be raining very heavily in order to describe it as "raining cats and dogs." Idioms can be metaphorical, polysemic, opaque or transparent, and are often colloquial or cultural.
These phrases or sentences are peculiar, enigmatic expressions of a given language. At face value, based solely on the literal definitions of the expression's words, idioms often make no sense. Usually, the expressions have a hidden cultural context, such that only a native of the language and of the culture from which the phrase arose can understand its meaning. They are a significant field of study for theoretical linguists and educators of foreign languages.
Many idioms are metaphorical. Several adages are derived, for example, from the metaphor of time as a currency. Some of the metaphors are obscure analogies, but others may be broadly universal. "Spending time" with children is a phrase that can probably be understood in any language translation.
The most common type of idiom are polysemes. They are words — often verbs — and phrases with multiple, somewhat related meanings. An example is the verb "run;" to "run with a smart idea" or "run a computer program" are related to, but quite different from running a foot race.
Although the distinction is not an absolute one, different types of idioms are categorized as either opaque or transparent. The determinant is to what degree the idiom's literal translation reveals, with some thought, its underlying meaning. "Leave no stone unturned," is a transparent idiom for searching thoroughly. The opaque German idiom, "to bite into the grass" might mean various things, but the expression becomes quite clear when explained that it means "to die."
Idioms are almost always colloquial or cultural. Americans are often uncomfortable talking about death, so the cryptic English idiom is "to kick the bucket." This exact same expression in Brazilian Portuguese, however, means "to give up, with emphatic drama." Both were born independently from their respective cultures, and have true meaning only within their local context.
The cultural depth of idiomatic expressions is to the extent that most native speakers of a language are rarely aware they are uttering colloquialisms. Some linguists and sociologists speculate that these inventions of language are a culture's way to differentiate itself — a code which outsiders cannot decipher. As such, idioms are often the most difficult aspect of a foreign language to both learn and comprehend.