A very important concept in cultural studies. All social and cultural relationships are pervaded by power. It holds society together, and also subordinates some people to others. Power is not necessarily bad, or good for that matter, but it has to be examined, and we have to ask why it is there, who gains and who loses, how it is produced and maintained, how it is reinforced in our systems of meaning.
Nietzsche is the first great theorist of power, but Michel Foucault is a more recent inspiration for its analysis.