It usually has to do with the concentration level of scents. Without knowing the actual concentration and level percentage, their hierarchy goes something like:
1. Perfume
2. Eau de Perfume
3. Eau de Cologne
4. Eau de Toilette
With perfume being the strongest and more concentrated out of the bunch, then with more added water (eau), the concentration level diminishes along with their lasting effect.
The way I see it, "perfume = everybody from all the way across the room, notices when you enter", "eau de perfume = people will smell you within 5 feet, and will keep smelling you after you're gone", "eau de cologne = people will notice what you're wearing by standing next to you, shaking hands or passing by next to them, fragrance will remain in clothing after wearing them", "eau de toilette = subtle fragrance, you will have to hug, kiss, or cuddle with somebody for you to have a good smell at it".
It also depends on the quality of the brand, and quality of ingredients used. Usually a good brand name, bought from a reputable source will adhere more to my remarks rather than a knock off.
Source: Own research while trying to get the right fragrance for myself. Ended up choosing "Eau de Cologne", great compromise between strength of smell, and long lasting effect.