What does the Nether Star in Minecraft do?

You can use the Nether Star to make a beacon, which offers buffs when activated. It also looks really cool, sending a beam of light into the sky.

I would check the source I listed below if you have any more questions regarding un-modded Minecraft. It is quite comprehensive, and updates extremely soon after each new patch or release. If you have questions about mods, use your favorite search engine to search for "(name of mod) wiki".