Most fragrances come in a line which contains a parfum or eau de parfum (EDP) as well as an eau de toilette (EDT) -- so what's the difference? The answer is concentration: the percentage of fragrant oils to alcohol.

Parfum:
A solution containing 15-20% fragrant oils and 80-85% alcohol. Because of the high concentration of oils, a parfum is the most expensive fragrance, yet it is also the longest-lasting, from 5-7 hours.

Eau de Parfum (EDP):
Contains 15-18% fragrant oils and generally lasts up to 5 hours.

Eau de Toilette (EDT):
Contains 4-8% fragrant oils and will usually last around 3 hours.

Eau de Cologne:
Contains 3-5% fragrant oils and lasts for only 1-2 hours.