Fragrances come in various types and concentrations, each with its own intensity, longevity, and olfactory characteristics. In the context of perfumery, the term "eau" is used to indicate the concentration or strength of a fragrance. The concentration of fragrance oils in a perfume determines how long the scent will last on the skin and how potent it will be. The term "eau" is derived from the French word for "water," and it is used to classify different levels of concentration. 

Parfum, also known as perfume or extrait de parfum, is the most concentrated form of fragrance. It contains the highest concentration of aromatic compounds, usually around 20-30%, making it the longest-lasting and most intense type of fragrance . Due to its high concentration, you only need a small amount for a long-lasting scent.

Solid perfumes are waxy balms infused with fragrance. They offer a portable and tactile way to apply fragrance directly to the skin.

Eau de Parfum is a popular type of fragrance with a concentration of aromatic compounds typically ranging from 15-20%. It offers a good balance between longevity and intensity, making it suitable for everyday wear. Eau de Parfum scents can last several hours on the skin.  

Eau de Toilette has a lower concentration of aromatic compounds, usually around 5-15%. It's lighter and more refreshing than Eau de Parfum, making it ideal for daytime use or in warmer weather. Eau de Toilette scents may last a few hours on the skin.

Eau de Cologne has an even lower concentration of aromatic compounds, typically around 2-5%. It's known for its  invigorating qualities, making it suitable for quick refreshment. Eau de Cologne scents are often used as a light body splash and may last for a shorter duration.

Eau Fraîche is a light and refreshing type of fragrance with a very low concentration of aromatic compounds, usually around 1-3%. It's designed for a short burst of fragrance and is often used as a body mist or splash.