Short answers, pulled from the story.
Photosynthesis is a system of biological processes by which photopigment-bearing organisms such as most plants, algae, and cyanobacteria convert light energy, typically from sunlight, into chemical energy. The energy is stored in the bonds of organic compounds like sugars, starches, and cellulose. Light-dependent reactions make ATP and NADPH, and the light-independent Calvin cycle uses them to fix carbon dioxide into carbohydrates.
Photosynthesis was discovered in 1779 by Jan Ingenhousz, who showed that plants need light, not just soil and water. Earlier work by Jan van Helmont, Joseph Priestley, and Jean Senebier in 1796 built toward the idea that green plants consume carbon dioxide and release oxygen under the influence of light.
Global photosynthesis captures energy at an average rate of about 130 terawatts, which is roughly eight times the total power consumption of human civilization. Photosynthetic organisms also convert around 100 to 115 billion tons of carbon into biomass each year.
C3 plants use RuBisCO to produce three-carbon acids directly, and over 90% of plants use this route. C4 plants, including maize, sorghum, sugarcane, and millet, first fix carbon dioxide as a four-carbon acid in mesophyll cells before releasing it to RuBisCO in bundle sheath cells. CAM plants such as cacti and succulents separate the steps in time, fixing carbon dioxide at night and using it by day, a pathway found in 16,000 species.
The ability to use water as the electron source in photosynthesis evolved only once, in a common ancestor of extant cyanobacteria, at least 2450 to 2320 million years ago. The excess oxygen these organisms produced oxygenated the Earth in an event sometimes called the oxygen catastrophe, making complex life possible.
Anoxygenic photosynthesis is a form of photosynthesis that does not produce oxygen. Some bacteria, such as purple bacteria, use bacteriochlorophyll to split hydrogen sulfide instead of water, releasing sulfur rather than oxygen, a form that dominated the euxinic Canfield oceans during the Boring Billion.