A normal pistol round that has been flattened at the nose and a hollow cavity drilled or formed into the front of the round to create a mushrooming effect on impact.

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"A hollow point, also called a hollow tip, is a bullet that has a pit, or hollowed out shape, in its tip, generally intended to cause the bullet to expand upon entering a target in order to decrease penetration and disrupt more tissue as it travels through the target. As a side effect, hollow-point bullets can offer improved accuracy by shifting the center of gravity of the bullet rearwards. Jacketed hollow points (JHPs) or plated hollow points are covered in a coating of harder metal to increase bullet strength and to prevent fouling the barrel with lead stripped from the bullet. The term hollow-cavity bullet is used to describe a hollow point where the hollow is unusually large, sometimes dominating the volume of the bullet, and causes extreme expansion or fragmentation on impact." "The Hague Convention of 1899, Declaration III, prohibits the use in warfare of bullets that easily expand or flatten in the body. This is often incorrectly believed to be prohibited in the Geneva Conventions, but it significantly predates those conventions, and is in fact a continuance of the St. Petersburg Declaration of 1868, which banned exploding projectiles of less than 400 grams, and weapons designed to aggravate injured soldiers or make their death inevitable. Regardless, any captured combatant who is found to be in possession of prohibited ammunition during an armed conflict can face very serious consequences, e.g., summary execution. It is partly for this reason, as well as humanitarian concerns, that NATO members do not use small arms ammunition which is prohibited by the Hague Convention. Despite the ban on military use, hollow-point bullets are one of the most common types of civilian and police ammunition, due largely to the reduced risk of bystanders being hit by over-penetrating or ricocheted bullets, and the increased speed of incapacitation. In many jurisdictions, it is illegal to hunt game with ammunition that does not expand, and some target ranges forbid full metal jacket ammunition. Some indoor shooting ranges prohibit the use of hollow points, as the rounds can create shrapnel." Source and further information: http://en.wikipedia.org/wiki/Hollow-point_bullet