From Diablo Wiki
A "build" is a character design, including the skills, traits, equipment, strategy, and everything else used to make that type of character work. Common, generic types of builds include casters, tanks, summoners, battle mages, and dozens more.
Builds can be powerful or not, depending on the type of build. Cookie cutter builds are generally very powerful and successful, which is why they become standarized and popular. Variants are builds that are not so common, often created for fun, or variety, or to give the game more challenge.
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