A flower is the reproductive structure of most plants. They are often brightly coloured and contain nectar to attract animals to pollinate them, once pollinated flowers develop into a fruit containing seeds. Because flowers are often attractive to look at and smell they are also grown for use as gifts and decoration.
Edosian Orchid were a type of flower native to Cardassia. Tolan Garak, and later his son Elim Garak, had a passion for the orchids. When the Orchid's flower was cross polinated with the flower of the White Star of Night it created a deadly toxic fatal to anyone who inhaled it. (DS9 novel: A Stitch in Time)
In 2370 Worf bought Deanna Troi a bunch of flowers, but dropped them as he walked into her quarters finder her and Will Riker engaged in a pillow fight, Deanna quickly collected the flowers off the floor, explaining the innocence of the situation. (TNG comic: "Strategy")
Later in 2370, when Jean-Luc Picard was visiting Boothby the Starfleet Academy gardener, Boothby picked out a flower from the Academy grounds’ flowerbeds for Picard as a parting gift. Unfortunately Picard dropped the flower moments later while attempting to avoid the phaser beams of a Tellarite assassin. (TNG comic: "An Inconvenient Truth")