The Nether Star is vanilla item obtained by killing the Wither boss. It is used in crafting end-game items, such as the vanilla Beacon, and in some mod items such as the Supercharged Gravity Gun, Inventory Module, Miniature Black Hole, and Tier 2 Nuclear Information Reader.
Since the star is used up in each recipe, you need to obtain one star for every recipe you wish to complete, meaning you must kill the Wither multiple times. Alternatively, you can craft two Stars from one and some other ingredients.
Aside from crafting recipes, Nether Stars are also used in some Thaumcraft 3 items such as the Wand of the Thaumaturge. Also, it can be converted into 10,000,000 EU in a Magic Energy Converter, enough to fill up an MFSU completely. Due to the cost however, it's not recommended that you do so.