Some perfumes use pheromones from the animal world. "Civetone", for example, comes from both male and female civet cats and musk comes from male musk oxen. Both pheromones are used to mark territory and to attract the attention of the opposite sex. Human urine has been found to contain a substance with a strong musk-like smell.
Men's urine contains twice as much of this substance as women's urine. The substance smells like a scent that women pay particular attention to during ovulation. There's a fatty acid in women's vaginal fluids that is similar to the sex attracting substances of other mammals.
A perfume's scent changes as time goes by. The reason is that certain scents in perfume evaporate at different stages. That's why the smells aren't all sensed at the same time.
Perfume manufacturers divide the process into three phases:
1) "The Main Scent" - the most fleeting substances that evaporate within an hour.
2) "The Heart Scent" - less fleeting substances that last for almost 3 hours.
3) "The Body of the Scent - the least fleeting substances that last up to 20 hours.
One perfume doesn't smell the same on two people because their sweat mixes with the perfume.