The difference between fragrance types is mainly the concentration of aromatic compounds.
Parfum (extrait)
About 15 to 30 percent aromatic compounds, usually the most concentrated.
Eau de parfum
About 15 to 20 percent.
Eau de toilette
About 5 to 15 percent.
Eau de cologne
About 2 to 3 percent.
A caution
These ranges overlap and brands set their own concentrations, so an eau de toilette from one house can be stronger than an eau de parfum from another. Test on skin and judge by how it lasts.
Source: Wikipedia: Perfume.
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