In sexually reproducing organisms, offspring receive two copies of each gene, one from each parent. When those genes show true dominance/recessiveness, the dominant trait is expressed (shows up) when there is either one, or two copies of the dominant gene. In order for the recessive trait to be expressed, two copies of the gene for the recessive trait must be present, one from the mother and one from the father. An example in humans is having a widow's peak (dominant). In incomplete dominance, if an organism is heterozygous (has the two different types of gene), both are expressed, to a degree. An example is the color of some flowers. Snapdragons that receive one gene for red color, and one for white, look pink.
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.
Codominance and incomplete dominance are two different types of traits. As you know, each parent contributes one allele for a trait (unless it is a trait controlled by multiple alleles). In a typical dominant/recessive trait there is a dominant trait T=tall t=short. T from mom and t from dad would result in Tt a tall plant. tt are the alleles for a short plant.
For codominance this is not the case. Both alleles are dominant. Crossing a black cow with a white cow results in a black and white cow. Both the black trait from the mother and the white trait from the father are dominant and the baby is black and white.
Incomplete dominance is when neither allele is dominant and a new trait is formed. For example, a red flower and a white flower result in a pink flower.