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 Boss a multitude of times which can become very aggravating. Alternatively, you can craft two Stars from one and some other ingredients.
RecipeEditOne of the ways to obtain nether stars: use the Tome Of The Alkahestto duplicate the nether star, at a cost of 1 diamond, 3 glowing water and 3 Wither Skulls. Therfore, if you do not wish to have your world blown up and have a overflow of diamonds, then by all means this is a good alternative. However, it may just be easier to create the wither, kill it with 3-4Holy Hand Grenadesand save 1 diamond, as well as not having to go to the trouble of creating aTome Of The Alkahest.
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.