Alkalinity is the capacity of water to neutralize acids - how much acid it can adsorb without changing the pH. It is a measure all alkaline materials in the water such as carbonate, bicarbonate, hydroxide and occasionally borate, silicate and phosphate. More alkalinity means that as acids are added the pH will drop more slowly. Water which is naturally alkaline occurs when water passes over rocks - like in springs - and picks up minerals which increases the alkalinity. The amount of Alkalinity in typical drinking water is 20-200 mg/L for typical drinking water.
pH (power of hydrogen) ranges from 1 to 14. Each point on the pH scale is a multiple of 100. A neutral pH is 7. pH indicates whether a liquid is more acidic (lower pH) or alkaline (higher pH). Pure water has a neutral pH of 7, while tap water has some natural variation depending on its mineral content. Most bottled waters are slightly acidic, and sodas and juices are even more so.
Every body fluid has a different pH. Blood is 7.4, stomach acid is 1 to 3, the large bowel is 5.5 and bile is alkaline to counter stomach acid.
Something that is often confusing is that when people talk about “Alkaline” water, they are usually referring to water with an elevated pH. They are not talking about water with increased alkalinity that will buffer the pH.