Codominant genes are both expressed at the same time if present in an organism. For example, if a person inherits a gene for type A antigen from one parent and a gene for type B from the other parent, that person will have type AB blood. In this case, both the genes for antigen A and B are expressed and the person will have both the A and B proteins present in their blood.
However, in incomplete dominance, the two genes "blend together" to form a different phenotype. For instance, in Japanese four oclock flowers, the gene to produce the color Red (R) and the color white(W) are both equally strong. Therefore, if a plant inherits a genotype of RW, this plant will not be red, nor white, but instead, a blending will occur, producing pink.