Glycolysis is an essential process in cellular respiration, converting glucose to pyruvate while giving a net of 2 ATP and 2 NADH. Pyruvate can then be fed into the link reaction if further ATP is required, continuing on into the Krebs cycle (as acetyl CoA) and the electron transport chain.
Glycolysis uses glucose as its starting material. The ubiquity of the pathway is likely due to photosynthesis, which produces glucose, being so prevalent. As photosynthesis was evolved in early bacteria, glycolysis came about around the same time to give non-photosynthetic cells a way to use the chemical energy produced by phototrophs. Glycolysis is used in plants, including in photosynthetic cells, to provide energy when it is dark (and thus photosynthesis cannot occur).
Glycolysis is used in both aerobic and anaerobic respiration.