The two most common cannabis plant types are Cannabis sativa and Cannabis indica. Less common is Cannabis ruderalis, which is used mostly by breeders to enhance their hybrids.

It was once believed that Cannabis sativa and Cannabis indica had distinct effects. The former was thought to produce more energetic effects, while the latter produced more calming effects. While some of this basic knowledge may still apply, now that growers have cultivated hybrid strains for so many years, references to the effects of pure species may no longer be relevant or helpful.

Today, there are hundreds of strains of plants- each bred for specific characteristics and often intended to produce specific effects for consumers.

Cannabis sativa 
This species can grow quite tall (up to 25 feet) and has long, serrated light green leaves. Cannabis sativa L, more commonly known as hemp is typically grown for industrial use and is regulated to ensure the plants do not contain more than 0.3% THC.

This species is generally faster-growing, bushier and up to six feet shorter than Cannabis sativa, with dark green leaves. 

This species is short, stalky and shaggier with light green leaves. It tends to be rugged and auto-flowering, which is good for breeders.

Hybrids
There is a lot of speculation about the varying effects of consuming predominantly sativa versus indica strains, but significant differences can occur from strain to strain. Many products are made from hybrid plants, which combine both species, as growers explore and cultivate cannabis to produce desired effects.