What is the difference between eau de parfum and eau de toilette?

The difference is fragrance concentration. Eau de parfum (EDP) holds a higher percentage of fragrance oils, so it smells richer and lasts longer, while eau de toilette (EDT) is lighter and fresher and usually needs reapplying sooner. Neither is 'better': EDT suits hot days and the office, EDP suits evenings and cooler weather. You can compare both formats at Perfumania.