Definition of concentration

Concentration is a way of describing mixture composition. While we will mostly use concentrations to describe water solutions, they can be used for any mixtures, not only liquid ones. Composition of alloys is often expressed in terms of compound percentage, the same happens with fertilizers. When we say air contains 1% of inert gases we are referring to their concentration as well. Other than percentages ways of expressing concentrations are used less often, but their presence is well established in some areas.

Molarities and normalities are used mostly in analytical chemistry as they facilitate stoichiometry calculations. Molality and molar fractions are useful in physical chemistry, as they don't change with temperature.

Percentages are garbage - only weight percentage makes real sense. However, percentages are the easiest to understand and solutions with concentrations given as percentages are also the easiest in preparation, so they are used quite often.