"The art of saving seed has been practiced by Gardeners long before there were commercial seed producers. In fact, most of the vegetables and flowers we have today owe their existence to the fact that these early Gardeners, with an eye for quality, saved the seed of their best plants, sowed them the next year, and in this way improved the species.
In recent years, the responsibility for maintaining and improving vegetable seed has been assumed by seed companies; however, it is still possible for home Gardeners to save their own seed. To do so successfully, they must be familiar with the basics."
Read here for the basics.