Alkaline substances are those which are basic, meaning they have a pH above 7. Substances with a pH of 7 are neutral and those with a pH below 7 are acids. Most common alkaline substances are cleaners such as soaps, detergents and shampoo. Alkaline substances have a slippery feel which is characteristic of cleaning products.
Ammonia is an alkaline substance that dissolves in water to make ammonium hydroxide, found in window cleaner. Ammonia-containing products are also used as plant fertilizers.
Baking soda is sodium bicarbonate, which is alkaline. It reacts with acid to product CO2 bubbles that make baked products rise.
Antacids are alkaline. Calcium carbonate and magnesium hydroxide are commonly used in antacids. Their purpose is to neutralize some of the acid in the stomach.
Lye is alkaline. It's a very caustic substance (sodium hydroxide) that's found in drain cleaner and is also used to make soap.