Reactive stains, or acid stains, penetrate the pores of the concrete that they are applied to. The hydrated lime that is present in the concrete then reacts with the chemicals in the stain, producing a translucent color. Since this is a chemical reaction, results will vary naturally, but you will get a color that is beautiful and quite close to the color of the stain you've chosen. Acid stains don't peel off or flake because they're a permanent part of the concrete once applied.
Non-reactive, or water-based stains, are applied in the same way that acid stains are, however they do not cause a chemical reaction in order to get their color. Instead, the stain penetrates the concrete itself, which then embeds the stain's pigments into the pores of the concrete. This allows for many different shades to be applied, however the results will vary like the acid stains because different concrete mixes will produce different results