Also called atrial natriuretic factor. As the name implies, it is released from the atria of the heart. It regulates sodium excretion by the kidneys, thereby also regulating blood pressure, which is really what it's there for.

It is a potent vasodilator, though whether or not it is implicated in system-wide control of arterioles is currently uncertain.

In the kidneys, ANP acts on the tubules, especially the distal tubule, to inhibit sodium reabsorption. It can also increase the glomerular filtration rate by dilating the afferent arterioles of the nephrons.

The cells in the atria are stimulated, (causing secretion of ANP), not by sodium concentration as one might expect, but by increased atrial distension. More blood volume will cause more pressure, which will stretch the atria more in the filling stages of the cardiac cycle. See also frank-starling relationship. While not directly related it operates on similar mechanisms.

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)