Eau de Parfum and Eau de Toilette carry different strengths of perfume. Eau de Parfum typically contains 15% aromatic compounds, while Eau de Toilette contains 10%. If you are looking for temporary yet cost effective product, then Eau de Toilette is a better choice. But if you are product with longer lasting scent, Eau de Parfum might be a better choice.
Eau de Parfum is composed of 10-20% aromatic compounds. This means that 10-20% of the perfume is made of perfume oils, while 80-90% is alcohol. The typical Eau de Parfum is 15% perfume.
Eau de Toilette is composed of 5-10% aromatic compounds, typically 10%.
Other strengths include Perfume Extract, with 15-40% fragrance. This is only generally found in top-of-the-line fragrance lines. Eau de Cologne has between 3% and 8% fragrance, while After Shave typically has 1% to 3%.
edit Difference in Scent
Eau de Parfum has a stronger and longer lasting scent than Eau de Toilette. It has strong heart notes that linger after the top notes have faded away and is the strength used in many designer perfumes.
Eau de Toilette only has top notes that fade relatively quickly. It is the strength found in many department store perfumes.
In this video Ben Bennett, host of welovebeauty explains the difference between perfume, eau de parfum, and eau de toilette:
edit Problems in Terminology
Different perfume houses will use slightly different definitions of Eau de Parfum and Eau de Toilette, meaning that their strengths might overlap, with the Eau de Toilette of one perfume house being stronger than the Eau de Parfum of another. However, the Eau de Parfum will always be a high concentration and so more valuable than Eau de Toilette within the same fragrance line.
As different brands use different concentrations, the only way to compare cost is to compare an Eau de Parfum to an Eau to Toilette in the same line.
In Chanel No. 5, the Eau de Parfum costs $115 for a 3.4 ounce bottle (or $33.82 per ounce), while the Eau de Toilette costs $90 for a 3.4 ounce bottle (or $26.47 per ounce).
In comparison, the Chanel No. 5 Parfum costs $260 per ounce, and the Body Spray costs $13.10 per ounce.
Current prices for some of the popular brands are available on Amazon.com.