Here is a quote from Hilary Black of the British Columbia Compassion Society: Indica and Sativa are the two main varieties of the cannabis plant used as medicine. There are many strains that are crosses of those two varieties. Within each of those varieties and crosses there are a huge number of individual strains, each with a different cannabinoid profile and effect.
As Indica strains have more chlorophyll than Sativa they grow and mature faster. Combining different Indicas, different Sativas or a combination thereof creates hybrids. The resulting hybrid strains will grow, mature and smoke in relationship to the Indica/Sativa percentages they end up containing.
The benefits of hybridization is that the breeder can attempt to create a breed that can fit exact needs, depending on the grower or end user. In todays market the hybrids dominate, as most strains have been crossed with each other at one time or another. Hybrid cannabis strains of note: Love Potion, Harmony, Thunderbud, Super Silver Haze, Timewarp, C99, Sour Diesel, Snowcap, Trainwreck, Apollo, Big Bertha, Skunk.
Pure Sativa cannabis is a rarity in the medical marijuana field as it is very hard to cultivate, especially indoors and also outdoors in the northern hemisphere (USA/Canada). Seeds of these types are also rare to find as vendors like to stock popular kinds and pure Sativas arent that popular and they come primarily from more equatorial regions (Colombia, Mexico and Southeast Asia). They are tall, thin plants, with much narrower leaves and grow a lighter green in color. They grow very quickly and can reach heights of 20 feet in a single season. Once flowering has begun, they can take anywhere from 10 to 16 weeks to fully mature. Flavors range from earthy to sweet and fruity.
Pure Sativa highs can often induce paranoia attacks and irregular heart beats so as a medicinal product Sativas are a rare choice. One of the positive effects of Sativas are their uplifting capabilities that may aid in depression and/or chronic fatigue syndrome. Examples include Acapulco, Burmese and Malawi Gold.
Although pure Sativas are quite rare, they are highly coveted by breeders who use their genetic stability to create designer hybrids.
These are Sativas that have been worked on by a breeder to fit special criteria such as exotic smells, tastes and the up high Sativas are known for. Often these hybrids contain some Indica genetics but appear mostly Sativa in nature. Hybrid Sativas are coveted as medicine for the motivational properties they possess, often without the paranoia associated with pure Sativa. Sativa hybrids rarely have any pain blocking attributes and shouldnt be used as such. These types have anti-depression and appetite stimulation qualities, so can be valuable as medicine in that regard.
Common Sativa hybrids include Haze, Kali Mist, Jack Herer, Willy Nelson.
Pure Indicas originally come from the hash producing countries of the world like Pakistan, Afghanistan, Morocco and Tibet. They are short, dense plants, with broad leaves that often grow a darker green, with more chlorophyll and less accessory pigments (accessory pigments protect the plant from excessive sunlight). After flowering starts they will be mature in 6 to 8 weeks and are better for indoor growing because they dont get as tall. The buds will be thick and dense, with flavors and aromas ranging from pungent skunk to sweet and fruity.
Afghani is by far the most ubiquitous strain and many modern strains incorporate these genetics. Indica plants have by far the most narcotic, body stone and pain blocking qualities so they are highly coveted by medical patients with any sort of pain. As a result of their higher CBD levels they are also helpful as a sleeping aid for those suffering with insomnia. The most common Indicas are Afghani, Pakistani, Hashplant, Herijuana, Hindu Kush and G-13.
In terms of medical value these are the best and most widely appreciated. Todays Kush strains are a prime example of these types and are easily the most popular. They tend to have many qualities that both grower and patient find to their liking, therefor the majority of medical marijuana is hybrid Indica. A few examples of hybrid Indica include: OG Kush, Master Kush, Purple Kush, White Rhino, Romulan, Blueberry, Dutch Treat, Grapefruit and Northern Lights.
(Citation comes from medicalmarijuana.ca.)