Official samples
These are usually small formats supplied by the brand or an authorized retailer, often as a spray vial or miniature container. The key advantage is original-source packaging.
Decants
A decant is fragrance transferred from a larger bottle into a smaller container by a seller or shop. Quality depends heavily on the source and handling.
Travel sizes
An official travel size is usually larger than a sample and smaller than a full bottle. It can make sense when you already know the scent but do not need the largest size.
When each format is useful
For a quick first impression, a small sample may be enough. For several full wears, a decant or travel size gives you more time to test.
Check size and source
Read the milliliters, packaging method and seller information carefully, especially when buying a third-party decant.