Please read the updated (and far more extensive) version of this cannabis breeding article here:
When you cross-breed two different (the more genetically different the better) marijuana strains, the resulting seeds have something that is known as hybrid vigor. Here's the wikipedia article on hybrid vigor if you're interested.
How do you create two genetically different strains for hybrid vigor?
You first breed a female plant with itself using a feminization method. You then take your self-crossed female plant and cross it with a highly inbred male plant of a different strain.
Hybrid vigor only seems to apply to that first generation of seeds. These seeds are known as F1 (first generation cross) seeds.
Subsequent generations after the first set of seeds don't show signs of hybrid vigor unless they also are crossed with different strains. Commercial Corn growers take advantage of this same mechanism by producing hybrid maize made from two different strains of corn; the resulting seeds have better yields than open-pollinated corn.
You can take advantage of this well-documented quirk of nature, too. With marijuana plants, the first generation of seeds from a hybrid cross produce plants that have about 25% higher yields than if you produced seeds from two plants of similar strains. Breeding two different strains of marijuana plants for F1 seeds (in order to get hybrid vigor) is often a more time and space effective way for the hobbyist grower to spend their time as opposed to trying to breed their own strains. As long as you keep crossing with plants that have huge different genetics, you will keep getting F1 hybrid seeds.
The other really important thing to remember when breeding two plants is to start with two really great strains first. You can easily find marijuana seeds to suit your needs and budget online that will produce great plants. You can even sometimes purchase F1 feminized seeds online so you can take advantage of the hybrid vigor without having to put in the effort of breeding your own plants.
The other problem with breeding is most people don't like having to deal with male marijuana plants (why?). However, you can actually easily create your own feminized (all-female) seeds at home by crossing two female plants. This means you never have to deal with males plants again, but can still produce F1 seeds or even create your own strains. This page has more information about cheap and easy ways to cross two female weed plants and produce your own feminized cannabis seeds at home.