Male and female pot plant differences.
Cannabis 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.
The timely identification of a male plant is crucial to the success of the harvest. If the weather is exceptional during the time a male starts producing its flowers and you missed seeing the first signs during your last visit, you could wind up with a lot of seeds and little of the fine herb. A female can either generate a large seedless bud, a large bud with a few seeds, or a large bud that is almost totally seeds. The first case is achieved by removing all the male plants before any of their flowers open. The second case occurs when a few male flowers have opened but you remove them before any more open. The third case occurs when you miss-time the flowering of the male. This can be devastating if you have big female plants because you could loose 90% of the smokable herb to seed production. This last scenario may not always be bad though. If you are short on seeds for the next growing season, it may be prudent to let one or two males stand and fertilize a portion of the females.
Pot plant gender indentification by Ed Rosenthal.
Good seeds are hard to come by, so if you have a strain you like, make sure to plan ahead and have at least a few hundred seeds for the future. The spotting of males is one of the most difficult of things to explain to a person that's never grown since it really takes careful attention to how the tops of male plants look at this stage of development. Even experienced growers will be unsure at times and will have to wait till the next visit to be sure. 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 bunch of grapes (little balls) but it will have no white hairs coming out of it. Females will have no balls and will have small white hairs.
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.
Found among Erowid's guides.