Once you accept that left and right are merely measures of economic position, the "extreme right" refers to extremely liberal economics that may be practised by social authoritarians or social libertarians.
Similarly, the "extreme left" identifies a strong degree of state economic control, which may also be accompanied by liberal or authoritarian social policies.
It's muddled thinking to simply describe the likes of the British National Party as "extreme right". The truth is that on issues like health, transport, housing, protectionism and globalisation, their economics are left of Labour, let alone the Conservatives. It's in areas like police power, military power, school discipline, law and order, race and nationalism that the BNP's real extremism - as authoritarians - is clear.
This mirrors France's National Front. In running some local governments, they reinstated certain welfare measures which their Socialist predecessors had abandoned. Like similar authoritarian parties that have sprung up around Europe, they have come to be seen in some quarters as champions of the underdog, as long as the underdog isn't Black, Arab, gay or Jewish ! With mainstream Social Democratic parties adopting - reluctantly or enthusiastically - the new economic libertarian orthodoxy (neo-liberalism), much of their old economic baggage has been pinched by National Socialism. It's becoming the only sort of socialism on offer. Election debates between mainstream parties are increasingly about managerial competence rather than any clash of vision and economic direction.
In the United States, the voices of dissent over unfettered market forces (ie extreme right economics) are heard from social authoritarians like Pat Buchanan as well as social liberals like Ralph Nader.
As an example, take a look at the ground that the main parties in England's 2003 local elections (May 1) occupied in reality. The difference between the BNP and the Greens in economics isn't great, but there's a huge gap on the social scale. Neither scale, however, reveals enormous distances between the Conservatives and New Labour.