Like pigeonwarrior said, it is the observable traits of an organism. Compared to a genotype, which is what your genes say. Phenotypes are very specific to the genes you have. In short, lets say you have B = dominant || b = recessive/
BB = You will see the dominant phenotype Bb = you will see the dominant phenotype bb =. you will see the recessive phenotype.
In short, the phenotype is really dependant on how many and where the dominant gene is!