For pollination to occur, a flower has to develop both the parts that produce pollen and the parts that receive it. These are the male and female parts of a flower, the stamen and pistil, respectively. The stamen has an anther at the top where pollen is created. The pollen grains contain sperm. The pistil, meanwhile, has a stigma at the top, there to receive pollen. The sperm make their way down the pistil to ovules, which, once fertilized, eventually become seeds. The ovules are contained within ovaries, which turn into fruit. With all the tools for fertilization ready, a flower activates its pollination strategy.
Some flowers use a self-pollination strategy, possessing both male and female parts so pollination can happen within the flower. These flowers are called perfect flowers and are considered bisexual. Some of these bisexual plants have stamens active at one time and pistils active at a different time so that cross-pollination can occur. Cross-pollination is pollination that happens between different plants, leading to genetically stronger offspring. To increase the odds of cross-pollination, some plants produce flowers that are either male or female. These unisexual plants might have male and female flowers on the same plant (monoecious plants) or all male or all female flowers on different plants (dioecious plants). Male and female plants must be planted near each other for pollination to occur.
Some flowers rely on the wind to accomplish pollination. These flowers tend to be small and occur in bunches. The wind picks up pollen, and, if all goes well, deposits it on the stigmas of other flowers, accomplishing pollination. Other plants have taken a more active role to ensure pollination. These plants produce showy flowers that attract the attention of pollinators like insects, birds and mammals. Some are shaped in ways geared toward particular pollinators. Tube-shaped flowers, for instance, might favor hummingbirds or long-tongued butterflies. Colors might be particularly attractive to certain pollinators--yellow and blue for bees. Markings might act like a bulls-eye, directing insects to where nectar awaits, the bugs picking up pollen on the way. Scents also attract. Most of the time, these scents are pleasant to us and other creatures. A fruity flower odor, for example, might be attractive to bats. Other odors, such as rotting flesh, are repulsive to us but are attractive to bugs like flies that visit carrion. These various strategies lure pollinators to a flower, where they get dusted with pollen. When pollinators move on to other flowers, they bring the pollen with them, where it rubs off onto the stigma of the newly visited flower, accomplishing pollination.