A flower is a plant's reproductive organ. Seeds are produced within fruit, and all fruit are formed at the bottoms of flowers.

Some flowers have female characteristics. They receive pollen, either by wind or insects, and create a fruit containing seeds. The female part of a flower always has at least one style, a tubular structure that extends outward from the base of the flower. At the top of each style is a stigma, a little button-shaped cap that is covered with tiny pores to admit pollen. When insects visit the flower, they try to crawl inside to reach the sweet nectar. Insects, such as bees, often have pollen stuck to their bodies, and when they brush pollen on the stigma, pollination occurs.

Pollen travels down the style through pollen tubes, to the very bottom of the flower. The female flower has an ovary there, where the pollen combines with ovules (like egg cells in animals) to create brand new baby plants. In most plants, each grain of pollen fertilizes one ovule, and creates one new seed which is a 50/50 genetic combination of the mother plant (with the flower) and the father plant (where the pollen came from).

The ovary expands to form a fruit, the flower dries up and falls off, and the seeds mature. You can often see the shape of the tiny fruit at the base of the flower - the ovary is really the unfertilized fruit.

Some flowers have male characteristics. They don't have a style, or an ovary. They don't produce a fruit. But they make pollen. The male part of a flower always has a number of stamens, slender supports with anthers at their tops. The anthers produce pollen.

Some flowers have both male and female organs. This may seem strange to us, but for plants it's normal. When stamens and styles are both present in a flower, we call that flower complete. Plants are divided into families on the basis of their flower shapes. If one member of a plant family has complete flowers, then you'll find that all members of that family do too.

Complete flowers
Every flower has both male organs (stamens and anthers) and female organs (stigma, style and ovary). Some are self-compatible; able to pollinate themselves without any help (e.g. tomatoes, lilies, petunias). Some are self-incompatible, requiring pollen to move from flower to flower (e.g., apples, pears).
Incomplete or monoecious flowers
Each plant has some female-only flowers and some male-only flowers. Pollen must move from the male flowers to the female flowers in order for seeds to be produced. (e.g., corn, pumpkins).
Dioecious flowers
Plants are truly all-male or all-female. Some plants only have male flowers, and some plants have only female flowers. You need two plants (one of each) in order to get fruit, and the fruit only appear on the female plants (e.g., holly).