A room is made up of surfaces. At its barest it is a floor, walls, and a ceiling. The floor, by nature of gravity, is where most items go. The walls are more like canvas, letting you decorate the nature of the space, coloring gaps that might otherwise be blank. However you can create many more surface areas throughout the space using a variety of methods.
The most basic horizontal surface is the tables. There's a huge variety of tables, ranging from large dining pieces, to tiny accent stands. Smaller tables are often used to display a lamp, a vase, or some sort of artistic piece. Coffee tables are fairly functional, and can frame a dining room, giving it a natural structure around which to arrange seating. In a kitchen, these surfaces can serve a dual purpose, giving the room a social element, while also providing an extra place to prepare culinary dishes.
Table Shelves
Shelves are another way to create horizontal surfaces in a room. They can be used as a place to display decorative items, giving you another element to alter within the room. They can also be used for organizational purposes, allowing you to keep books, magazines, or even functional items in a room in a logical and organized manner.
A mantel is sort of a centerpiece shelf. It is hung in such a way that it will dominate the visual space, overlooking a room like a crowning jewel. Usually the most impressive decorative piece in the room will be displayed there, since it is so prominent.
Desks, dressers, and cabinets are all horizontal elements in a space, although these tend to have a much more strict functional purpose. Console tables are similar, in that they are placed near a chair or other appropriate location where a beverage can be rested, or a remote can be stored.
The horizontal surface options in a room are endless. It is possible to create a complex array of varying heights throughout a room using these different decorative elements. The important thing is not to clutter a space, but rather let it flow naturally as you design its interior.