When someone from Britain speaks, Americans hear a "British accent"...

[Luis M. Gonzalez]

Grant Edwards wrote:
> That depends on the accent.  I believe that's probably true for
> the educated south of England, BBC, received pronunciation.  I
> don't think that's true for some of the other dialects from
> northern areas (e.g. Liverpool) or the "cockney" accent.

What's exactly the "cockney" accent?
Is it related to some place or it's just a kind of slang?
I'm not sure, but I think that I read somewhere that it is common in
some parts of London, and that it is a sign of a particular social
class, more than a regionalism. Is that true?