Dictionaries are resources (printed or online) that includes definitions, meaning in different languages, synonyms, word usage and grammar, and often provide information related to pronunciation. Some dictionaries are specialized in specific field of knowledge.
Oxford Dictionaries: Oxford Dictionaries focuses on current language and practical usage. The English site provides free access to the largest current English dictionaries and thesaurus as well as helpful tips on grammar, usage, spelling, and more.
Cambridge Dictionaries: Cambridge published its first book in 1584 and is the oldest publishing house in the world. Cambridge University Press has been publishing dictionaries for learners of English since 1995. Cambridge Dictionaries Online began offering these dictionaries completely free of charge in 1999.