Tabulate area

One of the simplest analysis tools is in the Spatial Analyst toolbox, in the "Zonal" toolset, is the Tabulate area tool.  Within the analysis extent and window, it computes the area for each value of a raster.  That is, for landcover it will compute the area for each landcover class, or for 5-meter imperviousness it will compute the total area that is pervious or impervious.