This tutorial describes mechanisms for separating ions in a plasma device with respect to their atomic or molecular mass for practical applications. The focus here is not on separating isotopes of a single atomic species but rather on systems with a much lower mass resolution and a higher throughput. These separation mechanisms include ion gyro-orbit separation, drift-orbit separation, vacuum arc centrifugation, steady-state rotating plasmas, and several other geometries. Generic physics issues are discussed such as the ion charge state, neutrals and molecules, collisions, radiation loss, and electric fields and fluctuations. Generic technology issues are also discussed such as plasma sources and ion heating, and suggestions are made for future research.