There are three kinds of seeds: Open-Pollinated, Hybrid, and Genetically-Modified (GM or GMO).
OPEN-POLLINATED seeds ("OP" in the seed catalogs) can be heirlooms, but not necessarily. These seeds will produce plants like their parents, and if you collect and save the seeds, they will produce plants of the same type next year. Plant A + Plant A = Plant A. (ORGANIC seeds are simply OP seeds that are certified as grown without chemical fertilizers or toxic herbicides or pesticides.)
--- OP plants are the ones you would want to collect seeds from (even if they aren't heirlooms). While the variety as a group is practically identical, there are minute differences in every seed, called 'genetic diversity', the lifeblood of the seed world. The more plants are identical, the more susceptible they are to disease and insect damage; the more diverse they are, the more likely at least some will survive an attack or adverse conditions.
Here is a ETS link on
Seed Saving HYBRID seeds ("Hyb. or F1 or F2" in the seed catalogs) are plants of the same species (like tomatoes) that have been deliberately crossed by breeders to produce a new variety of plant by taking pollen from a specific male flower and dusting it onto a different variety female flower. The high-tech tool for this tends to be a small artist's paintbrush. Plant A + Plant B = Plant C. If you collect seeds from the fruit of Plant C and plant them the next year, there are absolutely NO guarantees of what you will get*, other than it will be of the same species (tomato). Many/most of these offspring will revert to resemble a parent or grandparent or GGP. So, to get more Plant C seeds, you have to breed Plant A with Plant B again. While the hybrids you buy are bred by man, they are natural. Mother Nature creates hybrids every time a bee or fly transfers pollen from one plant to another that is receptive to that pollen.
--- *My experiment: I love Sungold tomatoes. I knew the theory of hybrids (which it is), but I collected the seeds of a Sungold tomato anyway, and saved and planted the seeds a few years ago. The resulting plants (3) were bigger than a Sungold, the fruits were gold like the Sungold, but slightly larger, but the fruits were only edible if a person was desperate, flat and kind of sour, a far cry from the original. But my chickens, ravening feathered piranha that they are, thought they were fine. My dog (the Belgian Harvester) declined, and continued to pick from the Sungolds.
GENETICALLY MODIFIED ORGANISMS (GM or GMOs) are plants that are developed in a laboratory using genetic engineering. Scientists incorporate the DNA of insects, fish, plants, germplasm, or insecticides. These plants are in NO WAY natural. At this point in time, they aren't really available to the home gardener, just commercial growers. There are many questionable issues with these, which you can look up on your own, if interested.
So, if you want to save seed, you have to use Open Pollinated seeds.
If you don't care about saving seed and just want some fairly foolproof seeds with specific characteristics, hybrid is fine.
Sue