Phenotype is the complete set of an organism's observable characteristics: morphological (appearance), developmental, biochemical and physiological properties. These characteristics are the manifestation of the individual's genotype, or genetic makeup, and are influenced by enviromental conditions (e.g., identical twins do not look or behave identically).
For example, consider the Punnett square below, which shows the four possibilities for the inheritance of the Mendelian factor corresponding to flower color (purple vs. white) in pea plants. A plant only has two different phenotypes ("purple flowers" and "white flowers") but it has three different possible genotypes, depending on whether it is homozygous dominant, heterozygous, or homozygous recessive for the factor.
The phenotype includes traits that can be visible directly (e.g., pigmentation) or viewed indirectly by some technical procedure (e.g. human blood types). Some researchers include behavioral characteristics into phenotype and even the products of behavior (e.g., the form of a bird's nest, spider's web or beaver's dam).