Language of any culture is a result of a long historical "process" and in this process, each dialect competes to be ahead of others to be the most widely used one. Over the time national standards are established as a result of "ethnical distribution "political domination", "cultural adaptation" and "migration". When these political variation, cultural and educational needs require a national standard usually the language of the capital city, which is adapted as a national standard language.

Consequently, spelling conventions are established and received pronunciation becomes the criterion of good, correct pronunciation also correct patterns of vocabulary and grammar become "institutionalized"
Extreme variations from the standard language will fail to communicate the real message. What has all this information got to do with dictionaries? Dictionaries are in fact one of the most important devices to standardize the language. This occurs by creating a standard in meaning and usage. So a dictionary is socially the mirror of its society. All socially accepted conventions or refused ones can be derived from the content of the dictionaries. Usage labels are an important means to achieve this end

Types of Dictionaries

The variety and number of English dictionaries has grown rapidly especially in United Kingdom. The most well-known publishers are Chambers, Collins, Longman, Cambridge and Oxford University Press.
These publishing houses owe great deal to the computer technology because it enabled them to compile word "databases" more quickly. This resulted is a wide range of dictionaries for very different, specific fields which would meet the needs of different groups of people with different purposes, the "most active elements" of the important "sectors" in life.

I would like to present the essential characteristics of a learner's dictionary. The basic question is who is a learner, and what is in a learner's dictionary?

An overseas learner's dictionary is written primarily to satisfy the needs of any learner around the world. Most of the learners' dictionaries involve contemporary, authentic spoken and written English. They emphasize the importance of context and collocation. As we shall see later, all learner's dictionaries guide you to choose appropriate words or meanings in particular situations. Easy-to-understand pronunciation symbols, variety of examples, are very important components of a dictionary. There are two conventions in ELT dictionaries: the first is to give information about the entries to indicate how a word "operates" in English grammar.  The second is to give model sentences to illustrate usage which is clear, easy and useful to comprehend.
Such dictionaries are essentially designed to improve the learner's vocabulary and to help them in constructing their own sentences. The power of it lies in its complete and clear definition of an entry. They are seen as the sources of the 'best' language and provide an authoritative guide to usage.

Dictionaries as commercial products:

Dictionary-making is an art and it is worth some money so it is a commercial product. The publishing houses created special teams to compile and write a dictionary. This effort will result in considerable revenue.  Publishing houses invest large sums of money, employ many experts and spend money on advertisements. The market is essentially enormous and diverse; and producers endeavour to meet the needs of that market.  English Language Teaching is a very important market which is gradually developing by innovations in teaching methods and text books. Learners need reference books by which they can solve a problem they face while reading a text or writing an essay. International Learner's Dictionaries are of great value for learners to refer to. In these overseas markets, there are numerous dictionaries for a variety of audiences ranging from beginners to advanced learners.

The four major dictionaries which are widely known around the world are published by Collins, Oxford University Press, Cambridge University Press and Longman, and include invaluable information which most of the learners are unaware of. Users are ignorant about extra information in dictionaries because they can not give up the traditional idea of a dictionary, which should mainly be used to find meanings or spelling or sometimes pronunciation.  Each dictionary has unique characteristics which may serve learners in a far broader sense. For example, Collins COBUILD gives very detailed information (Grammatical usually) in an extra column while Oxford's Advanced Learners and Cambridge University's Dictionary for overseas learners of English provide learners with distinctions between synonymous words, grammar of certain words, level of formality, informality and British and American variations.