A steroid hormone, C₂₁H₂₈O₅, produced synthetically and by the adrenal cortex glands. It is the chief regulator of sodium, potassium, and chloride metabolism, thus controlling the body's water and electrolyte balances.

Aldosterone stimulates sodium reabsorption by the cortical collecting ducts in the kidneys. When present, approximately all of the sodium is reabsorbed, maintining blood pressure. A low sodium diet or loss of sodium in large amounts can stimulate secretion of aldosterone, via angiotensin II.

The entire system is sometimes called the renin angiotensin aldosterone system.

Aldosterone is also related to potassium regulation, as mentioned above. Increased plasma potassium directly stimulates the adrenal glands to produce aldosterone and release it into the blood stream. This causes increased potassium secretion in parallel with sodium reabsorption, via Sodium-Potassium ATPase in the cortical collecting ducts of the nephrons in the kidneys.
Aldosterone also aids in acclimatization to warm climates by reducing sodium lost via an increase in reabsorption in the sweat glands.
Aldosterone is released during stress to help retain water and sodium in the body, in case of hemmorhage or sweating.

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These are my interpretation of my lecture notes, but I may have used some references from Hole's Anatomy and Physiology (Shier, Butler, Lewis) and Human Physiology (Vander, Sherman, Luciano)