If you look at a cross section of a piece of wood, the rings are immediately apparent. These rings are alternating bands of light and dark colored wood which were laid down as the tree grew. This variation in the composition of the wood from one place in the sample to another is what makes it heterogeneous. Wood, then, is heterogeous through and through as is typical for natural substances. A few natural substances are homogeneous , that is, their composition is uniform.
Air is an example of such a substance.
If you look at a sample of air, its composition is the same from place to place within the sample. There are no areas of different color or density and this uniformity extends even to the microscopic level. Of course, if there is a fly in the sample, it is technically heterogeneous since the fly is in one part of the sample and not in another. And we could have heterogeneity at the microscopic level if there is dust suspended in the air. So the distinction between heterogeneous and homogeneous substances is a relative one, with wood and dirt and flies at one end and clean air at the other.