yay! i can answer that one! in a classic Mendelian gentic situation (menaing, idealized), the difference between a dominant/recessive trait and a codominant trait is as follows:
okay, so, if it's a dominant/recessive trait (again, in a classic situation), then there are two possible phenotypes (ways that the animal can look), and three possible genotypes (genes that that animal can have to make it look that way). so, let's just say the trait is albinism (Tremper, Rainwater, whatever, so long as it follows the Mendelian model). so, an animal that has an AA genotype isn't albino. And animal that has an aa genotype is albino. and here's the dominant/recessive part: if it's got Aa genes, the non-albino genes are dominant, and the animal is not an albino, and doesn't look (externally) any different from the AA animal.
so, if it's a co-dominant trait, there are three possible phenotypes. i don't know which traits with leos are codom, so i'll pretend there's another kind of albino (i'll call it the 'marla' albino- hehe), that is co-dominant. so, if the animal has MM genes, it's not an albino. if it has mm genes, it's a marla albino. and if it has Mm genes, the two traits are mixed. i think there are technically two different terms for how they can be mixed. but one way is for the animal to be like a checkerboard, one square albino, one not (get it?). the other way (which i think is what codominant usually means), is for the two traits to be evenly blended together. so an Mm genotype would i guess be sort of salmon-colored. i hope that helped! 
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marla
keeper of: axolotls, catfish, ferrets, leopard geckoes, oriental fire-bellied toads, and sugar gliders