If you are starting with two stable "breed true" strains then your seeds will be just like the parents if not better. Even if you selfed "self fertilization" an unstable female plant there is a 50% chance your seed "s1" will not end up like the parent. Google mendel's experiments for more info. An example of a breed true strain would be northern lights, white widow, or any older strain that has been around for awhile. If you find two different pheno types and cross them you will end up with a hybrid "F1". If it is a good match you will have what is called hybrid vigor, the seeds you make with this will be better than the parents. However dont go in thinking your going to make some awesome f1 hybrid seeds. It takes thousands of plants, techniques, and most of all patience.