All languages appear in forms (variants). Every language is an army of dialects. The two dominant varieties of English are the British English (BrE) and the American English (AmE). They are because they are the originating varieties (native speakers). There are other varieties apart from these two. All other varieties derive existence from these two.
In other words, other variants are upshots of these two dominant varieties. English Language is not the same all over the world. Nigerian English is a little different from South African English. Thus, English Language can be categorised into three: the inner circle, the outer circle and the expanding circle.
The Inner Circle: These countries have English Language as their native language (mother tongue). They constitute the core source of English Language. They include UK, USA, New Zealand, Australia and Canada. They are called the “Norm-generating Circle”.
The Outer Circle: These are nations where English has been adopted as the official language though not their native language. Countries like Nigeria, India, Kenya, Ghana, and the Gambia fall in this category. They are known as “Norm-developing Circle”.
The Expanding Circle: They are countries that have chosen English as an alternative language because of its relevance in the world and its globality. Countries like Russia, France, Korea, and Japan are in this class. They are also known as “the Norm-dependent Circle”.
All the language variants have mutual intelligibility as their common feature in that a speaker of variant can understand the speaker of the other variant. With comprehension gone, the variants become different languages rather than dialects of a language.
As implied in an earlier paragraph, the British English and American English are the standard varieties, dominant and likely used, widely acceptable, prestigious, and adopted as official languages. They are also used in important domains/contexts (as medium of instruction, in media, for documenting history, etc). This however does not undermine the growth and importance of other varieties as a language can never be detached from it socio-cultural background which influences the language use in every community, among people of different classes and professions, and in every nation.