Cross-pollination is when pollen from one plant is transferred to another plant of the same species. The resulting offspring from cross-pollinated plants are more vigorous and seemingly more disease resistant. Plants produce nectar which attracts insects to them so that the flowers can be pollinated. There are many insects that are attracted to this nectar and in the process pollinate the flowers. Bees, as a group however, are unique in that the nectar is their base source for carbohydrates and the pollen is their base source of fat and protein.
Bees are the primary pollinators of many plants.
The honey bee population is down at the present time due to traceal mites. It is impacting on gardens and agriculture. The best way to attract these various pollinators to the garden is through the use of nectar producing plants. It varies with the region and with the season. A good bet is to talk with your local agricultural agent to learn which plants do the best job of attracting pollinators in your area.