Since at least Plato’s time, many words have been spoken and written in answer to that question. In recent years, they have usually taken the form of “Justice is just [fairness, equality, utility, etc.].” A recent (and outstanding) contribution to the discussion is Anthony de Jasay’s Justice and Its Surroundings. My review is now online, so you can read why I’m so very enthusiastic about this book. (Adobe Acrobat required; you’ll have to scroll down to the bottom of the PDF page for the start of my review.)
The book is available from (in order of inexpensiveness): Laissez Faire Books and Amazon.com.
I strongly encourage people interested in questions of justice (and political theory generally) to buy and read Justice and Its Surroundings. One of the advantages of the book is that readers can pick out a few chapters and learn something important about game theory, social choice, and other issues, but without feeling obligated to read the whole book.