Unlike most plants, cannabis is a dioecious plant, that is to say, it features specimens of separate sexes (female and male). Female plants are best for producing buds while males are in charge of pollination. Unless you're a highly experienced grower capable of recognizing growth patterns during the vegetative phase, it isn't until the flowering that you'll be able to discern your plant's sex.

• Female plants develop a small pear-shaped bag from which protrude two hair-like stigmata, usually white in colour. Later on, these little bags will start to form resin-coated clusters that will grow and turn into buds.
• Male plants also feature a tiny sack, but without any protruding stigmata. You can even see that the flower is made up of five sepals, which are an essential part of the formation of the flower's calyx, sustaining the petals. Afterwards, male plants begin to display sacs of pollen, forming a cluster that hangs down. When they develop enough, about one or two centimetres, they will open up and drop the small granules of pollen.

More info: How to tell whether a cannabis plant is male, female or a hermaphrodite