AquaMarkers use water based pigmented inks. These have the advantage of being water soluble - they can be easily blended, even when dried, giving the effects of water-colour paint but with the convenience of a marker.
They are particularly suitable for popular craft activities. Being water based, they can be applied to rubber stamps and then easily cleaned away. And since pigmented ink is acid free and light fast, it gives the excellent colour longevity required by Scrap Bookers.