You start of with homozygous strains. These are considered true breeding strains, and will show little variation in phenotype. When you start mixing true breeding strains you end up with heterozygous strains. The phenotypes in these will vary a little more than the homozygous parents. A heterozygous strain is considered to be a hybrid. An F1, F2, F3 etc are all hybrids.
F1 hybrid - is the first generation of a cross between any two unrelated seed lines in the creation of a hybrid. F1 hybrids can be uniform or variable depending on the parent stock used.
F2 hybrid - is the offspring of a cross between two F1 plants.
A seed bank will have a description of a strain and its characteristics. For F1 seed stock, most of the plants will remain true to that description, with little variance. F2 stock will vary more, your get maybe 3-4 plants out of 10 seeds that will be true to the seed bank description. The rest will be a little different in their phenotype, producing different flowering times, different yields and different tastes and smells. Without this kind of experimenting, new combinations of genetics (and therefore new variations of marijuana) would never be found.
| 
