The % by volume that you see on the bottles is the amount of alcohol that is in the fragrance blend. Basically perfume companies 'pad out' their fragrances with alcohol, firstly to make them go further, and also because otherwise they would be incredibly strong to wear! A perfume with strong smelling essence ingredients will have more alcohol with it than one which is more delicate.
So exactly what is the difference between eau de toilette, eau de parfum, and the other fragrance forms? Here is a generalized list of the different concentrations available:
This consists of a 15-30% concentration of perfume essence and is the strongest fragrance available. This means that not only do you need to use less, but also that the fragrance lasts longer, typically for up to 6 hours, depending on skin type.
Parfum De Toilette
This is usually between a perfume and an EDP in strength but a few companies use the term to describe an EDT.
Eau De Parfum (EDP)
This is lighter than Perfume, with an 8-15% concentration, and so is less expensive but still has long lasting characteristics, from 3 to 5 hours.
This is a lighter more delicate fragrance, with a 4-8% concentration of the essence, and is therefore most suited to everyday use. It is normally less expensive than Eau de Parfum, and lasts for 2 to 4 hours.
This is often referred to just as Cologne and has the greatest dilution of fragrances with a 3-5% concentration of perfume essence, lasting up to 2 hours.
Eau De Toilette (EDT)
This has the greatest concentration of essence generally available for men and hence lasts the longest, up to 5 hours. It is often available in either a splash or a spray.
Less strong, it is the most popular choice and lasts for 2 to 3 hours. Aftershaves are kinder to the skin than EDT's. Those with very sensitive skin should use aftershave balms.