It can be as simple as growing unknown badseed and giving it your own name. Or it can be as complex as breeding a female of one strain to a male of another, which may take generations of backcrossing, to accomplish a stable entirely new strain.
In common with orchids and prize roses, Cannabis seeds strains are grown and crossbred by breeders to create new cannabis breeds with different characteristics. The most important characteristics manipulated through crossbreeding are size of plant, bud to leaf ratio, yield, taste, effect and acclimatisation to western growing conditions.
There are two main varieties of Cannabis found in the world: Cannabis sativa and Cannabis indica. Cannabis sativa (the ganja variety) is a large "pine-tree like" plant with light green foliage. It is indigenous to Mexico, Columbia, Thailand, India, and Africa.
Cannabis Indica (the hashish variety), on the other hand, is a smaller plant with wide, dark green foliage. It matures earlier and produces more potent, fatter, resinous flowers. Cannabis indica is indigenous to the high northern mountain ranges of the Afghani Hindu Kush, Pakistani Kara Korams, Russian Pamirs and Indian Himalayas.
There are many strains of each of the Cannabis varieties.
Until the last few decades there had been very little crossbreeding of these Cannabis varieties because of this natural geographic segregation. However, in the 1960s and 70s seeds from different strains of the two varieties were collected from around the world. The strains were crossbred by fertilising isolated flowers from one variety with pollen from a strain of the other variety, creating offspring strains that were superior to the parents in terms of resistance to disease, yield, and levels of THC (the psychoactive ingredient in Cannabis), and could be grown better in northern latitudes.
A term used to describe strains produced by crossbreeding two different "true-breeding" strains.
True-breeding strains will produce true to type if reproduced from their own seeds, so different generations will share similar characteristics. True-breeding strains are either:
traditional land races that have only interbred with similar strains and so have almost identical genes
Hybrids that have been created by breeders but then continually inbred until their characteristics are stabilized from generation to generation. There are very few strains of stabilized hybrids as the process takes a number of generations, over several years, to achieve.
"F1" denotes that the seeds or plants have two true-breeding parents. The crossing of F1 hybrid strains produces plants whose characteristics won't be the same as the original true-breeding parents, though they may share some. Plant quality is lost because the variation in genes is extended and making it less easy to foresee characteristics. The process is endless; F2s can be bred with other F2s creating F3s, with a further loss of quality.