Linear Fragrances
Some perfumes stay almost the same from the moment you spray them until they fade away. These are usually called linear fragrances. The notes may become softer over time, but the general smell and character do not change much. This can be a good thing if you really like the opening of a perfume and want that same smell to stay with you for most of the wear.
Evolving Fragrances
Evolving fragrances, on the other hand, change more clearly over time. The opening can smell completely different from the dry-down. You may first get citrus or fresh notes, then more floral, spicy or aromatic notes in the middle, and finally warmer woody, amber or musky notes in the base.
Don't Judge Too Early
This is also why you should not always judge a perfume only from the first few minutes. Some fragrances have an opening that you may not like very much, but after 20 or 30 minutes they can become completely different and much more enjoyable.
Depth and Complexity
Evolving fragrances are also known for having more depth. Many niche fragrances are quite deep, meaning it can be difficult to fully understand what they smell like from the first sniff. Designer fragrances, in general, tend to be less deep and easier to understand right away. The depth of a fragrance can be seen as one of the things that shows its complexity and quality, but this is not something everyone necessarily wants.