An idiom is an expression whose meaning is different from the meaning of its constituent words. Idiom comes from the Greek idios, which means personal. It originally meant "speech peculiar or proper to a people or country." These days we use idiom for a specialized vocabulary.
Idioms exist in every language. Knowledge of idioms is an important part of comprehension of the English language. They are often metaphorical and make the language more colourful. People use them to express something more vividly and often more briefly. They serve as an image or mental picture.
Idioms are mostly used in description of people, their appearance, character, mood, actions or behavior. Many of them are also applicable to things or places. A few examples of idioms are “as black as a crow” which refers to something that is jet-black, “as busy as a bee” which refers to someone who is very busy etc.