What are hypertonic solutions?

A hypertonic solution is one in which the concentration of some solute outside of a cell is greater than that inside of the cell.

One example of this can take place when a freshwater fish is placed in salt water. Because the concentration of salt in saltwater is much higher than that inside of the fish, water will move via from the fish (usually through the gills) into the water in an effort to equalize the concentration. Unfortunately for the fish, this results in rapid dehydration, followed by death.

Poor fish.

Here's a helpful image comparing different types of solutions:

This video shows onion cells losing water because of osmosis. The slide was originally made using distilled water, then drops of concentrated salt were added to the edge of the cover slip. The salt diffuses under the cover slip and plasmolysis begins at 0:48.

OK.