Flowers are a unique evolutionary development of a group of plants called angiosperms. Angiosperms developed during the time of the dinosaurs, successfully using flowers to reproduce themselves with beautiful diversity. Even though the flowers of angiosperms can look quite diverse in form--from the angular bird of paradise to the simplicity of the buttercup--all flowers have up to four of the same kinds of parts. All are used in reproduction.
Carpels are female flower parts that are found in the center of a flower, often merged into a single part called a pistil. Both female and androgynous flowers have pistils; androgynous flowers also have male parts.
A pistil is roughly shaped like a bowling pin, with a bulbous bottom, which is the ovary, and a bulging top, which is the stigma. The stigma catches pollen, which is deposited either by wind or creatures. Pollinators such as bees, birds and bats pick up pollen on visits to other flowers.
Once the pollen is deposited, sperm contained within it travels down the pistil through the style, a tube bridging the stigma and ovary. When the sperm reaches ovules contained in the ovary, fertilization is accomplished. Once fertilization occurs, the pistil produces fruit and seed, which then grow into new plants.
A flower's stamen is the male part of a plant and is possessed by male or androgynous flowers. Composed of a filament plus an anther, the stamen makes pollen. The filament holds up the anther, which is specific location where pollen is produced. This puts the anther and its pollen in the path of pollinators, who brush by as they pass.
Flowers usually have several stamens arranged in a circular pattern between the petals and the pistil, if the flower has a pistil.
Petals and Sepals
Though petals and sepals aren't strictly necessary for a flower to reproduce, they do play an important role in flowers that possess them. Sepals and especially petals attract pollinators. Indeed, you can often tell the flowers that aren't wind pollinated by how showy, nectar-filled and scented they are. All these devices are geared to get creatures to visit a flower and either pick up or deliver pollen. Because petals and sepals don't actually play a role in fertilization, they are called accessory parts.
Sepals are leaflike, protective structures, often brown or green. They enfold a flower bud before it blooms, and are found on the very outside of the flower. All the sepals together are called a calyx. Petals and sepals as a unit are called the corona of a flower.