Citrus - also named hesperidics - includes fresh fruits like tangerine, orange, bergamot, and lemon.
Most of the time, zest is the part of the citrus fruit used in perfumery. Traditionally, citrus notes were considered the most important ingredient of the traditional "Cologne", and are key to build an "Eau Fraîche".
They convey freshness and bring a spark to a fragrance. They are also very volatile (they evaporate quickly) and are considered top notes (the ones you smell first, at the time you apply the fragrance). In a fragrance, they should be paired with less volatile notes, which then take over to ensure long lastingness to the perfume.
Citrus Marine (inspired by D&G’s Light Blue Masculine) and Citrus Neroli (inspired by Tom Ford’s Neroli Portofino) are good examples of citrus fragrances.