Cannabis strains are either pure breeds or hybrid varieties of Cannabis, typically of the species C. sativa and/or the putative C. indica. Varieties are developed to highlight a specific combination of properties of the plant or to establish marketing differentiation. Variety names are typically chosen by their growers, and often reflect properties of the plant, such as taste, color, smell, or the origin of the variety. Cannabis strains commonly refer to those varieties with recreational/medicinal use, which have been cultivated for high cannabinoid content. Several varieties of cannabis, known as hemp, have no recreational value (low cannabinoid content) and have been traditionally been grown for their fiber, oil, and seed.
Variety ambiguity 
A variety may refer ambiguously to different forms of cannabis:
- Clone-only variety – A cannabis grower may grow a cannabis seed into a plant and find that this plant is unique in some way. The grower may make genetically identical clones of the plant and distribute these. A clone is the only way to propagate the exact genetic makeup that makes a variety unique, however, growing conditions greatly affect the plant and the final consumable product.
- Stable seed variety – For a cannabis breeder wishing to develop a new variety, the process is complicated and time consuming. It involves selectively choosing male and female cannabis plants and breeding them over the course of multiple generations. The final generation's seeds will have been stabilized by the breeder on the specific attributes chosen, though some genetic variation still exists among the seeds.
- Unstable seed varieties – While these can be produced more quickly, plants grown from these seeds may have widely varying characteristics. Reputable seed shops will not distribute unstable seed varieties, though some amateur growers might. Third-party growers may produce unstable derivatives from well known varieties and misleadingly call them by their true variety name.
- Wild varieties (landraces) – Some varieties, such as Colombian and Thai refer to cannabis plants found growing wild in certain regions. Typically, these plants are used as bases for the production of more specialized varieties (e.g. G-13 or Haze).
Additionally, black market Cannabis dealers may distribute marijuana that is misleadingly called by a variety name. For example, Skunk and G13 may be used, but a lower grade may actually be sold.
Major variety types 
The Cannabis genus is typically considered to have two species, Cannabis indica and Cannabis sativa. A third species known as Cannabis ruderalis differs from the other two species in a few key ways. C. ruderalis is very short, produces only trace amounts of tetrahydrocannabinol (THC) and flowers independently of the photoperiod and according to age.
Pure sativas are relatively tall (reaching as high as 4.5 meters), with long internodes and branches, and large, narrow-bladed leaves. Pure indica varieties are shorter and bushier, have wider leaflets, and are often favored by indoor growers. Sativas bloom later than indicas, often taking a month or two longer to mature. The subjective effects of sativas and indicas are said to differ, but the ratio of tetrahydrocannabinol (THC) to cannabidiol (CBD) in most named drug varieties of both types is similar (averaging about 200:1). Unlike most commercial drug varieties, indica landraces often consist of a mixture of plants with varying THC/CBD ratios. The relatively high CBD to THC ratio typical of hashish produced in regions where these landraces are grown (including Afghanistan and Pakistan) is useful for treating insomnia.
Jack Herer (sativa-dominant)
In addition to "pure" indica, sativa, and ruderalis varieties, hybrid varieties with varying ratios of these three types are common. For example, the White Widow hybrid is purported to have about 60% "indica" and 40% "sativa" genetics. These hybrid varieties have combinations of traits derived from both parental types. There are also commercial crossbred hybrids which contain a mix of both ruderalis, indica and/or sativa genes (these hybrids are usually called autoflowering varieties). "Lowryder" is the most famous auto-flowering hybrid and retains the auto-flowering characteristic of ruderalis plants, while also producing usable amounts of THC/CBD. Autoflowering marijuana varieties are considered advantageous by some growers due to their discreet size, short growing periods, and the fact that they do not rely on a change in light schedule to determine when to flower.
Variety naming 
Varieties are often named by the breeder or grower to differentiate one from another. In competitive legal markets, such as in Amsterdam, there is significant pressure to create unique varieties that dominate the market. This results in a number of distinct variety names that may refer to very similar cannabis.
Likewise, when a variety becomes popular, many breeders and growers may produce variations of the same variety using the same or similar name. For example, Sour refers to a subset of sativa-dominant Cannabis strains.
Breeding new varieties 
Breeding involves pollinating a female cannabis plant with male pollen. This will happen naturally. However, the intentional creation of new varieties typically involves selective breeding in a controlled environment.
Often male plants, once identified by their ball-like stamen, will be separated from female flowers. This prevents accidental fertilization of the female plants, either to facilitate sinsemilla flowering or to provide more control over which male is chosen. Pollen produced by the male is caught and stored until it is needed.
The seeds produced by a germinated female will be F1 hybrids of the male and female. These offspring will not be identical to their parents. Instead, they will have characteristics of both parents. Advanced techniques can stabilize certain characteristics.
A common technique to stabilize a cannabis variety is called "cubing", in which the breeder will seek specific traits in the hybrid offspring (e.g. greater resin production, tighter node spacing, etc.) and breed said offspring with a parent plant. The same traits are sought in the new inbred offspring, which are then again bred with the original parent plant. This process is called cubing because it usually repeated across three (or possibly more) generations before a variety can be considered at least somewhat stable.
Seed shops sell both pure varieties that have specific aspects stabilized as well as unstabilized hybrids that may be of questionable quality.
See also 
- Small, E. and A. Cronquist. 1976. A practical and natural taxonomy for Cannabis. Taxon 25(4): 405–435.
- Greg Green (2001). The Cannabis Grow Bible (4th ed.). p. 47.
- Hillig, Karl W. and Paul G. Mahlberg. 2004. A chemotaxonomic analysis of cannabinoid variation in Cannabis (Cannabaceae). American Journal of Botany 91(6): 966-975. Retrieved on 22 February 2007
- Cervantes, Jorge. Marijuana Horticulture: The Indoor/Outdoor Medical Grower's Bible. Van Patten Publishing. ISBN 1-878823-23-X.
- Auto-flowering Cannabis Strain: Best light schedule?