Cannabis plants reproduce by cross pollination which allows seed production. There are numerous cannabis seeds out on the market. You can generally divide these into three groups:
Regular Seeds: the plants from regular cannabis seeds can both be male and feminine. When they are cultivated in good circumstances, you are more likely to grow feminine plants.
Feminised Seeds: genetically altered so that they produce feminine plants in at least 95% of the cases.
Autoflowering Seeds: come from indica or sativa plants which were crossed with the ruderalis plant. The result of crossing the ruderalis plant with an indica or sativa plant is a fast blooming variant.