So what is it? In short, it's a field of studies based on the theory that whites invented the idea of biological race in order to oppress other people and benefit from unearned privileges.
To understand the theory in more depth I'd like to comment on an essay written by a whiteness studies advocate, Damien Riggs.
A) Race as a social construct
Riggs believes that race is a social construct rather than a biological reality.
On the face of it, this is a very odd assertion. After all, there do exist people who are white as a matter of human biology - tens of millions of them.
So why would Riggs hold to the "social construct" theory? The answer is, I believe, that the idea of race as a social construct fits in well with liberal political theory. It makes sense in terms of ideology, even if it appears to be at odds with observable reality.
The intellectual orthodoxy of our times is liberalism. Liberals believe that we become human when we are free to choose for ourselves who we are to be. Liberals, therefore, don't like the idea of a "biological destiny". They don't like, for instance, the suggestion that our biological sex influences who we are, as this is something fixed that we are born into rather than determining for ourselves.
Similarly, it makes sense within the terms of liberalism to deny the biological reality of race, and to prefer instead the suggestion that race is merely a social construct, something which humans have made and can therefore readily unmake.
There is one further step in the logic of whiteness studies. In theory, liberals could apply the social construct idea equally to all races. They could argue that all races are mere constructs to be overthrown in favour of a universal individualism.
Whiteness studies is more partisan than this. Theorists like Riggs don't stop at the suggestion that all races are social constructs. They go on to ask a more specific question of why humans invented whiteness. The answer they give is that it's to allow some people to get power over others. Whites exist because whiteness allows them to be privileged oppressors.
This last claim has some particularly unpleasant consequences, which I'll discuss later. For now, though, it should be possible to understand Riggs' summary of his own field of study:
Whiteness is seen as a thoroughly racialised project that aims to legitimate the authority of certain groups over others by drawing on a legacy of 'biological' explanations of race ... Whilst this approach starts from an understanding of race as a social construction, it also acknowledges the very concrete ways in which race shapes experiences of oppression and privilege.
The theory of social construction is not without its contradictions - as those advocating whiteness studies are only too aware.
Theorists like Riggs wish to deny the real existence of race and to persuade us that race is a fictional category. At the same time, though, their central focus is on "whites" as a real category of privileged oppressors. In fact, in trying to highlight racial privilege, one of their aims is to try to get whites to be more conscious of their "racialised" existence - they don't want whites to be race blind.
So whites are being told: you don't exist as a race, but as a racial power category you do exist.
It's a difficult distinction to hold, and Riggs himself warns that:
it is important to recognise that in talking about race we run the risk of reifying race as a 'real entity'
(There's a couple of other important aspects of whiteness studies to discuss but I'll leave them till later.)