A bouquet is a decorative botanical item held in the off hand.
A bouquet is made up of several flowers, and some of the items listed are single flowers, but:
The word flower refers to a common and showy plant part, so flowers are often a visible part of the landscape. Many herbs are flowers as depicted by their icons, and often the resource node that is harvested is a flower object. Lasher mobs, from which parts are harvested, are plants with a flower where the head would be.
Because of this fairly egregious set of ambiguities, the name of this article is "Bouquet", referring to the function of being floral, decorative, and carried, which is far more specific than "Flower".
[15-30] 
The model name on the chart is not official. The point of the Model column is that bouquets with the same model name use the same graphic object model, with coloring (skinning) variations, when the item is displayed on the character. There are currently four bouquet models that a character can equip.
Display[]
Press the "z" key to toggle displaying your equipped items. (With a ranged weapon equipped, you may have three display options, in which case it will step through them.) One of the display options will show the bouquet being held in your off hand.
Non-functional bouquets[]
These cannot be equipped by the player character.
The 
[Daffodil Bouquet] quest item is for a low-level Alliance-only quest, 
[15] Delivering Daffodils. The quest text reinforces that it is conceptually a bouquet.
[Freshly Picked Flowers] pledge gifts are given out by non-guard tauren NPCs (primarily on Thunder Bluff) during the Love is in the Air seasonal event. These can be collected and used five at a time to create [Basket of Flowers] pledge gift collections. (These are in turn used to create [Thunder Bluff Gift Collections].) These floral items are not specifically called bouquets, and could conceptually serve some other unstated purpose.
The 
[Jannok's Rose] quest item was for the night elf rogue's quest, 
[10] The Apple Falls, that lead the character from the Dolanaar rogue trainer to the Darnassus rogue quest giver that gave the main level 10 night elf rogue quest.
Achievements[]
Looting a 
[Bouquet of Red Roses] or 
[Bouquet of Ebon Roses] drop completes the achievement [My Love is Like a Red, Red Rose].
Using a [Bouquet of Red Roses] to shower a nearby target with pink petals gives credit for the 
[Fistful of Love] and 
[Flirt With Disaster] achievements. (Using a [Bouquet of Ebon Roses] does not.)