Genetics
A plant's appearance, size, growth rate, and many other characteristics of strain are hard-coded into the plant through its genetics. Over time breeders, have combined desirable genetics to achieve new characteristics, such as auto-flowering.
Flowering
There are two categories that define how cannabis plants enter the flowering stage: Autoflowering plants that enter the flowering stage after a period of time, and photo flower plants that enter flowering based on photoperiod.