Sativa, Indica and Ruderalis
Cannabis is a plant that indigenously originates in Asia but its use and cultivation goes back far in human history and has long been established in Africa, Europe and America. It is a versatile and fast growing plant and can grow in a wide varieties of climates, though it prefers warmer climates with good sun exposure.

Though its proper name is cannabis hemp it is also known as Marijuana (a name which ironically comes from a slang term for tobacco in south and central America but was adopted by the US government as a recognised name for the cannabis plant).
Cannabis has 3 basic species, Sativa, Indica and Ruderalis, though many modern strains are actually hybrid mixtures with Sativa-Indica crosses the most common.
Though there is much debate on the subject there is no actual distinctive difference between cannabis and hemp. Hemp is nothing but another name for cannabis. The term hemp is usually reserved for male plants or strains very low in THC that are used for industrial purposes but the fact remains that hemp is another name for cannabis. Most products made from cannabis, like seed oil, paper, clothing material, fibre and food products are usually referred to as made from hemp. Prior to the prohibition of cannabis hemp was the most utilised plant for the purposes of producing fabrics, ropes and various clothing materials, including denim. Hemp remains the most efficient source for many products. Hemp paper is superior in quality to paper made from timber and can be produced in much larger quantities. Depending on the strain cannabis/hemp can produce 4 times as much paper as timber per acre and can be regrown every 6 to 9 months, as opposed to the many years required to grow the trees used for paper. Cannabis detoxifies and replenishes soil and its deep roots are very useful for preventing soil erosion. Cannabis can be grown in relatively arid lands and after a few crops can transform previously unsuitable land into land that can then be more suitable for other crops. Cannabis is environmentally friendly as it requires little to no pesticides and is considered a preferable source for clothing material and fabrics than cotton.
Cannabis can also provide wood, plastic and rubber like substances that are non toxic and can also be used to create bio fuels. Henry Ford famously built a car made almost entirely from cannabis hemp, even able to run on ethanol derived from cannabis hemp, and had a body made from hemp that was shown to be both lighter, more durable and stronger than steel.
In addition to the numerous materials that can be produced from cannabis hemp, which vary from high quality paper to building materials and a wide range of fabrics ranging from coarse fibres to silk like fabrics, cannabis hemp also produces seeds that are a complete food source. The essential fatty acids in hemp seeds are so complete that they represent the perfect ratio to meet the optimum nutritional requirements of humans. Hemp seed oil and hemp seeds supply everything the body needs from a nutritional point of view and is considered a "super food" that is ideal for optimum health.

Cannabis Sativa
Cannabis Sativa is identified as a more fibres plant and is characterised as a tall growing plant with leaves that have thinner more widely separated "fingers". Cannabis Sativas can grow to as tall as 6 metres. Though varieties do vary, most cannabis sativas are considered to be more THC/THCA dominant with very little CBD content. Though Sativas are also used for medicinal and recreational value they are more suitable for industrial use than Indica or Ruderalis due to their fibres, tall growing characteristics.
Though Cannabis Sativas are generally considered higher THC/THCA content, strains bred for industrial purposes have been selectively bred to be very low in THC/THCA content in accordance with legal restrictions on cannabis as a "narcotic". Male plants of all species have very low cannabinoid content so are considered ideal for industrial purposes.

Cannabis Indica
Cannabis Indica is a shorter species of cannabis and tends to be fatter in appearance. The leaves are also wider with fatter "fingers". Generally speaking Cannabis Indicas are considered richer in CBD than the THC dominant Sativa strains, though selective breeding has made high CBD strains rarer as THC is more sought after in recreational markets. Cannabis Indica is a shorter plant but is more bushy than either Sativas or Ruderalis and tends to yield more flowers than the other species.

Cannabis Ruderalis
Cannabis Ruderalis is the smallest of the 3 basic species. Ruderalis originates from Russia and is capable of suited to colder climates than the other species. While Sativas and Indicas flower in accordance with light cycles, which when grown outside is dictated by the seasons, cannabis ruderalis actually flowers in accordance with its age. This is known as autoflowering and is desirable for those who wish for fast flowering that can flower outside regardless of the season. Ruderalis are also known as "low riders" because they tend to be very small, a characteristic that can be very advantageous for many purposes.

Most strains available today are mixtures of the above 3 species. Indica Sativa crosses are the most common as the high THC content of many Sativa strains combined with the high yields of Indica can make for strains that are very desirable for recreational and medicinal purposes. Though Sativa Indica crosses are the most common the autoflowering and small size of Ruderalis makes them very desirable for many purposes.