Scenery is just texture replacement, they change the visuals so that everything looks better. Landclass files contain information about the landscape; it places towns, forests, lakes, rivers, roads, bridges and railroads where they should be, like in the real world. When working together you will get both great visuals as well as a realistic representation of how the real world is. You don’t have to buy landclass addons, but they do enhance the flying experience by quite a bit, especially if you enjoy flying over real world areas."
My understanding is this isn't quite accurate. Landclass has information about where to place textures such as town, forests, desert, wheat fields, etc. Lakes, rivers, roads, bridges and railroads is something different that is called landscape or vector. If you buy a landclass product from Scenery Tech or Orbx (for example) it won't have any of these things. However the Vector product from Orbx or Ultimate Terrain will have it. It's a technicality but can be important to determine which product to purchase.