Ruby's `::` operator, properly called the scope (or constant) resolution operator, used primarily to reference constants, classes and modules. It can also be used to call methods (`User::new.hello`), which works but is discouraged by RuboCop for regular method calls. Not fully interchangeable with `.`: `::` also performs constant lookup, so `Workplace::User` resolves a constant inside Workplace while `Workplace.User` is a method call.