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Idioms are expressions whose meaning is not obvious from the individual words.

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Collocation refers to the way that some words often occur together, for example a 'heavy smoker' or 'make a mistake'. 

Learning collocations is a good idea because they will help you speak and write English more accurately and naturally. What’s more, you will expand your vocabulary. Instead of using ‘big’ or ‘nice’ all the time, you will be able to choose words that fit the context better and describe meaning more precisely.
(Adapted from McCarthy, M., O'Dell, F. (2006) English Collocations in Use. Cambridge)

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