An idiom is a group of words whose meaning cannot be understood from the meaning of the separate words. For example, if you say that something is 'a piece of cake', you mean that it is very easy. Or if you describe someone as a 'basket case', you mean that they are crazy. There are literally thousands of idioms in English, and you can even find whole dictionaries devoted to idioms. There are also plenty of free websites where you can learn about idioms, their meaning and their origin. Here's a selection of the best ones …
101 English Idioms — A slide presentation I put together using images from the BBC Learning English Twitter feed.
1000+ Most Popular English Idioms and Their Meanings — Divided into categories such as Animal Idioms, Food Idioms, etc.
Common English Idioms and Sayings — English idioms and saying organized into themes.
Commonly-used Proverbs — Quizzes on more than 230 proverbs that most native English speakers know.
English Club Idioms List — With examples, notes and quizzes.
English Club Idioms Quizzes — Fun quizzes to check your understanding of English idioms.
English in a Minute — Learn common American English idioms from these VOA YouTube videos.
English Idioms and Idiomatic Expressions — Big list of idioms organized by theme.
English Idioms Illustrated — A beautifully illustrated free iOS app (see screenshot below).
English Idioms & Idiomatic Expressions — A dictionary of 3,865 English idiomatic expressions with definitions.
Idiom Site — An alphabetical list of idioms with definitions.
Idiomic — Fun, illustrated idioms each with a definition, example, and origin.
Idioms.in — Learn idioms with comprehensive meaning, examples and origin details.
Know Your Phrase — Meanings for many idioms and phrases, as well as information about where the expressions may have originated from.
Self-Study Idiom Quizzes — Dozens of multiple-choice quizzes on idioms, phrasal verbs and slang.
TalkEnglish.com — Useful idioms and phrases with an explanation, dialogue example, and other ways of saying the phrase or idiom.
The English We Speak — BBC Learning English audio series explaining common English idioms and sayings (archived).
The English We Speak — BBC Learning English audio series explaining new words an sayings in English (current).
The Free Dictionary — Idiom dictionary compiled from the Cambridge International Dictionary of Idioms and the Cambridge Dictionary of American Idioms.
The Idiom Connection — Alphabetical and thematic lists with quizzes for each letter or theme.
The Phrase Finder — 1,900 English idioms, phrases and expressions, with their meanings and origins explained.
Wikipedia — A selection of idioms, many of which link to individual articles.