Flowers are the reproductive part of flowering plants. All flowering plants are part of a group known as Angiosperm. Flowers' appearance can change wildly from plant to plant, but generally they consist of a group of colorful, ruffly oval shaped petals radially mounted around the end of a stem.
This tag should be used whenever a picture features a flower, for example, stuck behind one's ear, or on a flowering tree when in bloom. This also applies to characters who have visible flowers on them or are, themselves, a flower such as Flowey from Undertale.
Note, however, if the plant has not yet blossomed, use bud (flower) instead.