Co-dominance incomplete dominance.

[Albert Clark]

Don't worry you are not alone. For the most part the two terms are basically the same but used interchangeably. And you are correct. Co Dom is a visible mutation when a single gene at a allele is different than normal. A matched pair of this gene brings on a "super" form that looks different than the single. A allele is one member of a pair of genes at the same locus or location. The locus is the location of a gene / allele on the dna strand.