Plants can sense light by way of photons using the plant’s photoreceptors; the organisms or proteins of the plant that perceive light. The photoreceptors signal various biological changes from each wavelength of light affecting things like structure, growth, biomass, color, taste, aroma and so forth.

The photoperiod is the amount of time a plant is exposed to sunlight in a 24 hour period. There are three types of photoperiods: short-day, long-day and day-neutral. In indoor plant growth, the photoperiod can be controlled with lighting and does not necessarily correspond with physical seasons outdoors. 

Photoperiodism is the response a plant has to seasonal changes in day length. It takes into account the photoperiod and the plants react accordingly. It is often interchanged with the stage of growth a plant is in such as seedling, veg, flower or mother. For example, most plants are in a vegetative stage in the spring months due to the shorter amount of sun exposure (or the longer amount of night exposure), whereas, the summer is typically the flowering stage because the days are longer (and the nights shorter). In a veg stage, the plants are just beginning to produce their structure. During the flowering stage, plants are budding and reproducing in the form of fruits or flowers. You can control the photoperiodism your plants experience indoors with the amount of time and the type of lighting you use. Other things have an effect as well, such as temperature and nutrition.