... and how does it differ from a "dialect"? While we might be inclined to think of languages as mutually incomprehensible communication systems, with dialects being variations within a language, in fact it is a lot more difficult in practise to know where to draw the line: languages are more a continuum than a set of discrete entities. Romaine says that when comparing languages, one method is to compare word lists and to regard as dialects of the same language cases where there appear to be more than 80% shared terms; a matching rate of between 28-80% would be categorised as languages within a family. But "these measures of course tell us nothing of what the speakers themselves consider the status of these languages to be" (p5).
This continuum is especially noticeable in places situated near the border of countries who have different languages: people on one side of the border will often speak the same "language" as people on the other, although what they speak may not be regarded by someone in a different part of the country as being the "standard" version of either language.
Languages may be thought of as being distinct from each other by native speakers while in fact they are practically identical: political and cultural pressures come into play. For example, speakers of Urdu and Hindi can meet and converse without being able to tell which "language" the other is speaking, but since different writing systems are used for each language (Arabic/Sanscrit), they are perceived to be different languages.
So the distinction between "language" and "dialect" is very difficult to make in linguistic terms.
Standard English is usually defined in terms of grammar and vocabulary (not accent, although it is often associated with RP). It is "privileged" and therefore has a strong influence partly because it is the form typically used for writing, but also because it is perceived to be the "best" form - it has prestige value.
Often people adopt certain accents or dialectal features for prestige or social cohesion reasons, and so certain trends of pronunciation can be seen moving across geographical areas. This also happens when young people adopt forms different from their parents, etc..
See handout for examples of accent features