Since the early Cretaceous Period 140 or so million years ago, plants and their insect pollinators have co-evolved to form tight mutualistic relationships. The oldest known bee has been dated to the mid-Cretaceous period, around 100 million years ago.
There are over 100.000 species of pollinator around the world but bees are the only one the actively collect pollen.
Bees are closely related to wasps and ants - wasps and ants are both predators, with ants being basically wingless ground living wasps. Bees on the other hand evolved to make use of the newly evolving flowering plants, gathering pollen and nectar to feed their larvae. As the bees go from flower to flower to collect nectar and pollen, they transfer pollen between flowers, thereby pollinating them (pollen can be thought of as being like "plant sperm").
Some 250000 species of flowering plants and about three quarters of the eighty major worldwide crops depend on insects like bees for pollination.
Without bees many fruits and vegetables would become scarce and prohibitively expensive, or not available at all. What is more, bees pollinate flowers and plants that support other organisms - plants are at the base of food chains and they also provide shelter and habitat for many organisms. Biodiversity relies on plants - many of which in turn rely on bees.