Dominant Allele: An allele whose traits always show up in the organism when it is present. It's like adding hot sauce to any dish - no matter what else is there, you're going to taste it!
Recessive Allele: An allele whose traits only show up when no dominant allele is present. It's like adding sugar to tea; if there's lemon juice already added (dominant), you won't be able to taste sweetness unless there's no lemon juice at all.
Codominance: A situation where both alleles contribute equally and distinctly to the phenotype. Like a swirl ice cream cone with equal parts of chocolate and vanilla.