Most seeds you buy are F1, which is the seeds from the 2 parent strains.
You will have a bit of diversity in these. Some will be more like the mother, others like the father and a few different mixes of both. Like humans, some kids from the same parents can look completely different.
If you breed with two F1 plants from the same parents, you get F2 seeds.
In those you will have even more diversity and start picking up traits of the "grand parents" and get a much larger variety. These are usually the most fun to pheno hunt.
If you then breed with two of those F2 plants, you get F3 and those begin to be more stable (if the breeder does their job right in selections).
If you want almost exact same plants from seed, you will need to look for F3 or F4's.