Osmolality is a count of the number of particles in a fluid sample. The unit for counting is the mole which is equal to 6.02 x 1023 particles (Avogadro's Number). Molarity is the number of particles of a particular substance in a volume of fluid (eg mmol/L) and molality is the number of particles disolved in a mass weight of fluid (mmol/kg). Osmolality is a count of the total number of osmotically active particles in a solution and is equal to the sum of the molalities of all the solutes present in that solution.
For most biological systems the molarity and the molality of a solution are nearly exactly equal because the density of water is 1 kg/L. There is a slight difference between molality and molarity in plasma because of the non-aqueous components present such as proteins and lipids which make up about 6% of the total volume. Thus serum is only 94% water and the molality of a substance in serum is about 6% higher than its molarity. Except in unusual circumstances this difference is unimportant and the terms molarity to the molarity are often used interchangeably. Note that the size of the particle is unimportant so that a single ion, eg sodium, contributes as much to the serum osmolality as a single large protein molecule, eg albumin.
The osmolality of physiological fluids tends to be dominated by small molecules which are present in high concentrations. For example in serum, sodium, potassium, chloride, bicarbonate, urea and glucose are the only components present in high enough concentrations to individually affect the osmolality. Together these make up over 95% of total osmolality of serum. Large serum components contribute little to the overall osmolality. For example the molar concentration of albumin, the most abundant serum protein, is only about 0.6 mmol/L. Only a few exogenous compounds such as ethanol, methanol, ethylene glycol and manitol can be present in the blood at high enough quantities to significantly affect the osmolality.