Glowstone is a decorative block used for illumination. Glowstone is brighter than a torch. Glowstone emits a light level of 15, while a torch emits a light level of 14, making it slightly better but vastly more expensive.
It is good for decoration. They can melt Ice that is in range of it. When broken, they drop 2-4 Glowstone Dust. They can also melt Snow (layers).