Marijuana plants are either male or female. The male plants produce pollen which pollenates the flowers of the female plant, which once pollenized, produce seeds. If the female plant isn't pollenized (if there are no male plants nearby producing pollen), the flower/buds continue to develop and produce THC. Female plants which are not pollenized are referred to as sensemilla (without seeds).

Usually 40-50% of the plants are male.

Whats the Difference you ask?

Males are often, but not always, tall with stout stems, sporadic branching, and few leaves. Males are usually harvested except those used for breeding, after their sex has been determined, but before the pollen is shed. When harvesting, especially if close to females, cut the plant off at the base, taking care to shake the male as little as possible. This helps prevent any accidental pollination by an unnoticed, open male flower.

When a male enters the stage of flower development, the tips of the branches where a bud would develop will start to grow what looks like a little bud (little balls) but it will have no white hairs coming out of it. Females will have no balls and will have small white hairs.

Special thanks to Erowid