Three meanings:

The three classes, "L'Estats Generale" of French society - the word "estate" in this case means social position - clergy, landed gentry, the People. The idea of "Fourth Estate" is from Carlyle - "A fourth estate of Able Editors" - in his book on the French Revolution.

[link|http://www.worldwideschool.org/library/books/hst/european/TheFrenchRevolution/chap39.html|http://www.worldwide...ution/chap39.html]

Sometimes it is taken to mean the proletariat as distinct from the mass of people in the Third Estate.

It also refers to the English Parliament - and the sarcastic remark of Burke that the reporters in the gallery were more important in practice than the three estates of lofty figures (Lords Temporal and Spiritual, Commons) in the chamber.