There is basically no scientific or legal difference, because all cannabis is “Cannabis Sativa L.” In practice we can mention many elements that differentiate them: types of growth, effects,… Although there is no scientific difference between indica and sativa. In practice, there is a lot of differences between indica dominant cannabis strains and sativa dominant cannabis strains; they mainly come from the historical development of the strains and there ability to adapt to different environments.
Here one could have continue writing and published one of these classic paintings on the differences between sativa/indica. What more will you learn if you are told that sativa is larger than the indica, takes longer to bloom, leaves are different, or effects are specific to one or the other? It’s not that simple…
Indeed, there are many different origins between Sativa and Indica. But the crosses (called cultivars) for thousands of years are such, that it is currently extremely rare to find a variety 100% sativa or indica. In addition, there is a tendency to classify cannabis sativa, cannabis indica, and cannabis ruderalis as 3 different species, while all these species (including industrial hemp varieties) come from the single species cannabis sativa (one should say, cannabis sativa subsp. indica (subsp. being the abbreviation for subspecies).
What you see will not automatically be what you have (size, leaves, flowering time, smell, etc.):
- on the market, it is not uncommon to find the same variety classified as indica or sativa in two different breeders.
- depending on where and under what conditions your plant has sprouted, dry, humid climate, sunny or not, altitude, type of soil, ph, nutrients, will favour the substances it develops and therefore its morphology and effects
 Hillig, K.W. 2004. A chemotaxonomic analysis of terpenoid variation in Cannabis. Biochemical Systematics and Ecology 32:875-891.