As soon as you spritz a perfume onto your wrist and it reacts with your skin, you'll smell the top note, otherwise known as the opening or head note. The top note is like an introduction to the fragrance -- it might be fleeting, but it's your first impression, the first scent listed on the box, and often the selling point for the perfume. After about 30 seconds or so, you'll start smelling the middle and base notes and the fragrance will be noticeably different.
The top notes are meant to evaporate within a couple of hours, so they're usually made up of lighter oils. Citrus, herbal and lighter florals are common top notes, so you tend to notice lots of grapefruit, anise, lavender, chamomile and rose as you spray your way around the perfume department.
As we said, there's no official limit to how many ingredients go into each component of a perfume, but three seems to be the magic number for top notes -- at least that's how many are usually listed for a perfume. Chanel No. 5, a picture of which you'll find in the dictionary under "classic perfume," has top notes of ylang-ylang, neroli (which are actually more common as heart notes), and aldehydes (a synthetic compound). Joy by Jean Patou, another oldie but goodie, hits you first with peach, leafy greens and those aldehydes again.
Once the initial rush of the top notes subsides, the heart notes sneak in ...