mg/L: Milligrams per liter. Often used in water quality.
ppm: Parts per million. Equivalent to mg/L in water.
ppb: Parts per billion. 1 ppm = 1000 ppb.
mol/L: Moles per liter. Used for molar concentrations.
μmol/L: Micromoles per liter. 1 mol/L = 1,000,000 μmol/L.
μeq/L: Microequivalents per liter. Depends on ion charge.