Feminized marijuana seeds are the same as regular cannabis seeds but with just one very important difference. Every seed has a 99% chance to be a female plant. As you may already know, male plants only produce pollen and are no good to growers unless they want to make cannabis seeds. The female plant is the one that produces sensimillia, which means bud that contain no seeds.
Feminized seeds were first bred back in the early 1990's and they really revolutionised the way that growers operated. Before this, people would just use regular seeds which contained an average of 8 females to 2 males. Once flowered, the grower would then have to dispose of all male plants which was a huge waste of time and resources.