Traditionally, Eau de Parfum was more concentrated than Eau de Toilette, but today, it’s no longer just a question of concentration: Eau de Parfum and Eau de Toilette are now regarded as olfactory variations and are developed by the Perfumer under a common olfactory signature.
An Eau de Toilette will most often be associated with lightness, delicacy (or even freshness for men's fragrances), and will be particularly suitable for summer.
An Eau de Parfum, on the other hand, will be more enveloping, intense, and will generally last longer on the skin.