"We define ecoregions as relatively large units of land or water containing a distinct assemblage of natural communities and species, with boundaries that approximate the original extent of natural communities prior to major land-use change. Others have defined ecoregions as areas of ecological potential based on combination's [sic] of biophysical parameters such as climate and topography." (source: World Wildlife Fund).
World Wildlife Fund ecologists currently divide the land surface of the Earth into 8 major ecozones containing 867 smaller terrestrial ecoregions (see list). Many consider this classification to be quite decisive, and some propose these as stable borders for bioregional democracy initiatives.
The ecozones are very well-defined, following major continental boundaries, while the ecoregions are subject to more change and controversy. Accordingly the Wikipedia organizes ecology articles under each of the ecozones.
See also
Terrestrial ecoregion, marine ecoregion, fresh water ecoregion