We all know flowers as the colorful ornamentations God adorned plants with to attract pollinating insects and to bring beauty to our lives. Roses, lilies, pansies, petunias and countless other plants are all grown specifically for their eponymous flowers. However, almost all plants have some form of flower. Usually these look largely like what we would expect a flower to look like, if perhaps smaller than ornamental flowers. However, some flowers would not be recognized as such by most people.
The botanical definition of a flower is the part of the plant within with the gametes grow along with the surrounding perianth—the petals and sepals (those green leaves attached to the bottom of the petals). Again, this is essentially what one would expect, even if one doesn't think about plants' reproduction often. Their purpose is to attract pollinators to pick up pollen from the male flowers and deposit it within female flowers.
The main things to note here are that flower petals do not have to be big and colorful—grass flowers look like little more than hair—and that the ovary in female flowers is what eventually turns into botanical fruits once pollinated. Another thing to note is that some things we call flowers may actually be leaves or inflorescences.