Seeds from heirloom and OP varieties stay true to breed, so your next generation of plants will take on the same genetic traits of the plant they came from.
Seeds from hybrid plants, on the other hand, may take on the traits of just one parent, or even combine random traits of both parents to become a completely different plant from the one you previously grew.
Annuals are plants that go through one complete growing season before setting seed. They’re the most predictable, as you know the seeds will be ready for harvest within a few months of sowing.
Biennials, on the other hand, are plants that require two growing seasons to complete their life cycle. The first year, they focus their energy on vegetative growth. The second year, they reach sexual maturity and go into seed production.
When saving seeds, you should collect from your most vigorous plants with the most desirable characteristics — those that germinated first, produced fruit the earliest, produced the most fruit, or bolted last.