Step I Osmolality and Osmolarity?

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accepted109

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I've never really understood what either of these measurements meant. I've memorized the units:

Osmolarity = osmoles/volume of solvent
Osmolality = osmoles/ Kg of solvent

But can someone simply explain what each of these terms mean, and why they are important? Not a textbook definition, just an actual way to explain what they means in terms of fluid and why we use those values.