Think of marine mammals, which breathe air like we do but drink only salt water; and think of fish such as salmon which live part of their life in saltwater and part in freshwater. How do aquatic organisms deal with the different concentrations of salt in the water?
Marine animals use osmoregulation to regulate the use of salt and water inside of their cells. Considering that marine mammals only drink salt water, and salt water is high in solutes (it is hyperosmotic to the cells), water tends to be drawn out from the cells. (To be hyperosmotic, there must be less ‘free’ [salt-free] water and more solutes than the liquid in the cells.)
Fresh water on the other hand is low in solutes, and offers lots of ‘free’ water. This means that the solute-rich cytoplasm (of the freshwater fish) is hyperosmotic. (In other words, cells tend to absorb more fresh water). Aquatic animals face the danger of absorbing too much water and bursting their cells if their is no regulation.
Osmoconformers are organisms that maintain equilibrium (by being able to keep the concentration of solutes in their cells equal to that of the solutes in their environment). This ability is dependent on a stable water composition, and is carried out through the process of active transport (most of the time).
Osmoregulators are organisms that can regulate the concentration of solutes and water inside their body; due to this ability, they are able to tolerate a much wider range of environments (from fresh water, to salt water, to land) and are known to switch from one to another throughout their life cycle. Using active transport, excess chloride ions are removed and excreted, sodium follows the chloride closely after. Pumping out the salts allows the marine animal to maintain a hypo-osmotic state on the inside of their cells.