From Orbx :
Terrain Mesh - a grid of elevation data. (determines topography, hills, valleys, etc..)
Land Class - a grid that determines what class of land should be drawn at each point. (grass field, farm, sparse trees, dense forest, desert, city, small town,...)
Vector Data - a layer of lines (roads, rivers, power line) laid on top of the mesh, LC, & texture.
Not sure if its called Vector in FS9 or not.