Projection
Projection is how far a perfume spreads from your skin and how noticeable it is to people around you. A fragrance with strong projection can be noticed from a distance, while a softer one stays closer to the skin. Projection is usually strongest during the first hours and slowly becomes weaker over time. In general, fragrance projects better from warm skin than from clothing, so if you want your perfume to spread more naturally around you, spraying it on your skin usually makes more sense.
Longevity
Longevity is simply how long a perfume stays noticeable. Some fragrances disappear after a few hours, while others can last for most of the day or even longer. It is also worth knowing that longevity often works differently on clothes. A fragrance that lasts around 6 hours on your skin can sometimes stay on clothing for days. Your skin warms the fragrance and helps it evaporate and spread into the air, while fabric can hold onto the scent for much longer.
Sillage
Sillage is the scent trail a perfume leaves behind you when you move. It is not exactly the same as projection. Based on my experience, if a perfume has strong projection, its sillage is usually strong as well. However, it is also possible for a fragrance to leave a noticeable scent trail without having strong projection around the wearer.
Properties, Not Just Sprays
This may actually be one of the most useful things in this whole guide. Projection, sillage and longevity are properties of the fragrance. You can think of them almost like abilities that a perfume has. For example, if a perfume naturally has weak projection, spraying it 50 times will negligibly make it project much farther. What will mostly change is the concentration of the scent in the area where it already projects, meaning it will smell much denser and stronger, but not necessarily travel much farther. At the same time, no matter how strong your perfume is, if you want noticeable projection, you still need to spray enough. With only 3–4 sprays, most fragrances will not keep projecting strongly for a long time, except for some unusually powerful perfumes such as Black Afgano or Crush on Me.