With the Zonal Statistics Plugin you can
analyze the results of a thematic classification. It allows to calculate several
values of the pixels of a raster layer with the help of a polygonal vector layer
(see figure_zonal_statistics). You can calculate the sum, the mean value and
the total count of the pixels that are within a polygon. The plugin generates
output columns in the vector layer with a user-defined prefix.
Figure Zonal Statistics