While they were introduced in the original Diablo game expansion, Hellfire, runewords didn’t come back until Diablo 2: Lord of Destruction, which re-introduced Runes into the game.
Though establishing a build is part and parcel of any RPG set up, Diablo expands it further with the ability to create and set runewords, giving you additional effects. Following Diablo 2 builds allows you to get the most out of your chosen class.