Texturing

How would a person go about texturing an object that would need a diffrent texture for each surface? For example if there is a room, how would you make the floor have a floor texture, and the walls have a wall texture. If all the vertexes are in on array? Will I have to define the wall in a diffrent array then the floor? Another example of the problem I have is if I am making a terrain, how do I make the flat parts of the terrain a diffrent texture then the steep parts, if the terrain is all in one vertex array. The book I have (3d Game programming with C++ with john de goes doesn''t help at all) Am I doing this wrong? Anyhelp would be appreciated. ~wave