When growing to save seed, please try to match the seed saving difficulty with your level of gardening experience.
Easy seeds are great for beginning seed savers. These plants are mostly self-pollinating, so the risk of cross-pollination is minimal (but not impossible). Saved properly, they are reliably the same as the parent plant.
Advanced seeds are better suited for more expert seed savers. These plants are cross-pollinating and are very likely to cross-pollinate with other plant varieties, which can result in "mystery" plants. Saving seeds from advanced plants requires special precautions such as hand pollination, tenting, and other methods to ensure varietal purity.
See: A Beginner's Guide to Sharing and Saving Seeds