A flower is the reproductive structure found in Angiosperms, and has four basic components. In COMPLETE flowers, all 4 whorls are present (a whorl just refers to the collection of that structure on a flower; e.g. the 5 petals on an apple blossom are a whorl of petals):
||The male, pollen-producing sexual organ composed of a FILAMENT and an ANTHER
||The female, ovule producing sexual organ, composed of a STIGMA, STYLE, OVARY, and OVULE(s). The set of 1 or more fused carpels is a PISTIL. The gynoecium is the set of all pistils within the flower.
||Modified leaves, typically brightly colored, that surround the reproductive organs
||Modified leaves, typically green, that protect the flower before it unfurls and support it after it opens. Occasionally retained after fruiting (like the papery sheath on tomatillos)
The flower also consists of 2 non-whorled structures: the PEDUNCLE (or PEDICEL in an inflorescence) and the RECEPTACLE. The peduncle is the shortened stem The receptacle is the thickened part of the peduncle from which the flower parts develop. It is often located below the ovary (hypogynous, as in hypo: below + gynous: in reference to the gynoecium), but sometimes the ovary can be nested partly (perigynous) or fully (epigynous) within the receptacle.
Not all flowering plants have all 4 whorls on each flower. Any flower missing one or more whorls is INCOMPLETE. A male flower, which lacks at least the pistil, is called a staminate flower. Technically, the flower is part of the SPOROPHYTE and isn’t “male” since it’s the pollen grain (the male GAMETOPHYTE) that contains the male sex cells. If a whorl is missing, it means that it’s not necessary given the ecology of that tree. Wind pollinated flowers, for example, don’t need to visually attract the wind, and so tend to have greatly reduced or no petals at all. In some flowers the sepals are so modified that they look indistinguishable from petals (like the 3 outer “petals” on daylilies. When you can’t tell the difference between the two, we call the two whorls tepals.