What is the difference between alkalinity and pH?

Alkalinity and pH are only vaguely related by their interaction. However, they are not the same thing. Here is a quick summary:

Alkalinity is a measure of the buffering power of water. In the vast majority of cases, alkalinity is comprised of carbonate ion species (bicarbonate and carbonate) in drinking water. The pH of the water affects how much of each of those species is present. In drinking water, alkalinity is the derived from the concentrations of the bicarbonate and carbonate ions in the water.

pH is a measure of the hydrogen ion concentration in water. Since the hydrogen ion concentration is very, very low, chemists use the pH designation to give it a more usable form. For instance, pH 7 is equal to 0.0000001 moles of hydrogen ion per liter of water. Pretty clunky, yes? So pH 7 is a lot cleaner way of expressing that teeny concentration.

So, you should be more able to understand that alkalinity is based on the concentration of those carbonate species and pH is based on the concentration of hydrogen ions.