I assume you mean the difference between male and female blossoms. Basic and most visible difference: The male blossom has a long, leggy stem. The female blossom has a bulb at the base. This bulb can be small or large, and of varying shapes depending on the type of melon or squash. The bulb is the nascent fruit. All zucchini come from female blossoms.
If you look inside the blossom, the male has pollen covered upright stalks at the center. Inside the female blossom you'll see a small split bulb with a hole in the center, but no pollen.