The genotype is the unique combination of genes an organism inherits from its parents at fertilization. The egg and sperm each contain a haploid set of chromosomes and the zygote produced at fertilization contains a diploid set of chromosomes.
The phenotype refers to the observable traits in an organism. These traits follow Mendelian genetics principles along with effects on the phenotype due to environmental influence.
To demonstrate genotype vs. phenotype we can examine the trait of handedness in humans. Whether someone is left or right- handed is determined by a pair of genes with the gene for right- handed --dominant and the gene for left- handed-- recessive.
A person who is left- handed will know her genotype as well as her phenotype. The only combination that will produce a left-handed phenotype in an individual is if that person has two recessive left -handed genes in their genotype.
To contrast this, a person with a right-handed phenotype could have either of two genotypes--two right- handed genes (homozygous) or one right handed dominant gene and one left- handed recessive gene- (heterozygous). There is no way to tell a person's genetic makeup or genotype if they exhibit the dominant trait--the right handed phenotype by merely looking at them.
To conclude, phenotype is the trait we observe in an individual and genotype is the actual gene pair an individual inherits for that trait. Keep in mind that we have many recessive traits that are not expressed if they are paired with a dominant trait and therefore will not be seen in the phenotype of that individual.