I enjoy writing articles periodically about more serious science topics that play into developing a better understanding of cosmetics. This one is on tonicity. Tonicity refers to the affect of a solution on a cell. A solution  is a mixture of two or more substances in which one (a solute) is dissolved in the other (a solvent). So a solute is the substance that is dissolved in a solvent to form a solution. This solution is homogeneous, meaning that it is a single phase and none of the solute is visible in the solvent. Common solutes would be salt or sugar. A common solvent would be water.
That solute can exert a certain amount of pressure referred to as tonicity. This pressure can affect the fluid volume and the pressure in a cell by affecting the movement of water down its concentration gradient. Yes, water, as other molecules will move down it’s concentration gradient.
Tonicity has three classifications; hypertonicity, hypotonicity and isotonicity. These refer to the concentration of a solution in reference to a living cell (a membrane bound structure). 
Hypotonic –refers to a solution that is lower in solutes than that of the fluid inside of a cell.  As a result, water will flow across the cell membrane into that cell (down its concentration gradient) from the surrounding environment eventually causing the cell to swell and burst (c).
Hypertonic  – refers to a solution that  is higher in solutes than that inside of a cell. As a result, water will flow out of the cell into the surrounding fluid eventually causing the cell to shrink or crenate (d).
Isotonic – refers to a solution whose concentration of solutes is equal to the concentration of solutes inside of a cell. The water flows equally across the membrane in both directions causing no change in the volume or shape of the cell (a,b).
You may have heard of isotonic saline being given as intravenous fluid. Isotonic saline has the same concentration of salt as the blood. The isotonic solution is safe to put into the blood whereas a hypotonic solution would cause blood cells to burst and a hypertonic solution would cause blood cells to shrink which could tear their membranes and cause loss of fluid.
So what does this have to do with skin care? While water movement is important in skin care and we want to make sure we are allowing water to move into the skin not out of the skin, the strength of the stratum corneum is not so fragile that we need to be concerned with tonicity. Where it does come in important is in preservative properties. I’ve had several questions about salt or sugar being preserving and its ability to kill bacteria. While its true that salt and sugar have preserving ability, it is highly dependent on the concentration. When the concentration of salt or sugar is hypertonic to the bacteria water will be pulled out of bacteria cells destroying them. But if that salt or sugar concentration is diluted to isotonic levels the bacteria cell can then thrive. 
Hypertonic solutions are not typically used in skin care because they can have a drying effect on the skin, but they are used in some topical applications. For instance in nasal sprays hypertonic solutions will dry sinuses. Hypertonic solutions can also be used for cleansing wounds or even for exfoliation.
The principle of hypertonic solution for preservation is used somewhat in food applications such as with jams and pickles.  However, the concentration of sugar and salt is still not enough to protect the food so refrigeration or canning is also used. So too with cosmetics, tonicity can help contribute to preservative, but alone is not enough.