Incomplete dominance is when a dominant allele does not completely mask the effects of a recessive allele. For example, a red flower (RR) and a white flower (rr), whereby red is dominant and white is recessive. Incomplete dominance in this case would be a pink flower (Rr) which is a mix of the two alleles. In short, you see a combination / mix of both traits.
Codominance is when the dominant allele and recessive allele cannot block or mask each other. For example, a "red" cow (RR) and a white cow (rr), whereby red is dominant and white is recessive. Codominance in this case would be a roan cow which has both "red" and white hair. In short, you see both of the traits, as is, they don't mix.