I am working on a dedicated post on just this topic but I am going to respond here as well.
You have asked a good question that many who walk into my store ask.
Before going in detail, lets get the definitions right. There are two types of seeds open pollinated (OP)seeds and hybrid seeds. Hybrid seeds are those that are a result of cross pollination of two different varieties of the same plant. The first generation (or child) is F1 hybrid. OP on the other hand has its pollination within its own variety and with no human intervention(care is taken that other varieties of the same plant are not grown together).
When a hybrid seed is born, it carries the best of both the parents and there forms a vigor called hybrid vigor which results in better yield, tolerance and growth. When you grow a hybrid seed, you cannot save the seeds from the fruit as it might revert to one of its parent.. or other combinations of the characteristics which the grower/consumer is not interested in.
Hybrid seed is good, If you are going to grow commercially in a large scale. The varieties are bred for long shelf-life and transport friendly(thick skin).It has tolerance to diseases.
Is it safe for us? Yes (again it depends on what characteristics the parent plants posess). Using generally available Hybrid seeds are ok and they don't pose a thread at all. But we might slowly lose all the original varieties that we had and you cannot save your own seeds. If you want to just grow a vegetable and are learning how to grow. using a hybrid to start is good. if you want to try out different varieties of vegetables and try out the flavors. Using an open pollinated/heirloom variety is good. because you can save your seeds and use it the next season.
mother earth has been naturally making so many hybrids over these years..
So to conclude, if you can get access to open pollinated seeds, use them. if not, don't sweat.. start with hybrid seeds to get your gardening going and slowly you can move to saving your own seeds.
oh I almost forgot.. Hybrids are not GM seeds.
hope this helps