A tile is the smallest building block for an area. Tilesets contain a collection of tiles, and it defines a 10' x 10' space within the game world. Each tile can describe a different feature of landscape from a road, hill, tree, house, well, rock, water, chasm, or wall depending on the theme of the tileset being used.
NWN2 will use tiles only in interior areas. Exterior areas will use the new system of deformable height maps and paintable terrain.