The units of molarity are moles per cubic decimeter, written mol dm-3 or simply M. The cubic decimeter is identical to the liter; in many older textbooks you will find concentrations are written in moles per liter, written mol l-1.
2.0 mol dm-3 or 2.0 mol l-1 or 2.0 M all represent the same concentration.
Once it was normal for chemists to quote concentrations as (weight of solute)/(volume). However, the mole is now the most common way in chemistry of expressing the quantity of a substance, therefore molar concentrations are nearly always used.Note:
Be careful not to confuse molality and molarity. Molality's units are often represented by a lower case "m" whereas molarity's are often represented by an upper case "M".