Recessive: a recessive allele will be masked by a dominant one and not visible
Homozygous: if you have two of the same alleles for a gene in one persons DNA
Heterozygous: if you have two different alleles for a gene in in someones DNA
Phenotype: what allele is expressed as a protein
Genotype: what alleles you have in your DNA for a gene
Codominance: when two alleles have equal dominance (they will both be expressed)
By expressed I mean shown as in having brown hair is brown hair allele being expressed; and by masked I mean not expressed.