Redstone is for crafting and brewing. Redstone can be obtained by mining redstone ore with an iron pickaxe or better, by trading with priest villagers, or by killing witches. Redstone can be placed on the ground to make redstone wire, can be crafted into numerous redstone components and devices, or used in brewing potions.
Redstone Ore can be found in small veins, and each ore contains 4 or 5 dusts. They are pretty common deep under ground and are usally useless in vanilla.
Redstone has various uses in the Hexxit mod pack.
It is used in :