The main difference between an acid and a base is their pH level and how they interact with other substances. Acids have a pH less than 7 and can donate protons (H+) in a chemical reaction, while bases have a pH greater than 7 and can accept protons or donate hydroxide ions (OH-). This proton transfer is fundamental in neutralization reactions, where acids and bases react to form water and salts. The characteristics of acids and bases are critical in various fields, including chemistry, biology, and environmental science.