The Minié ball, invented by Claude-Étienne Minié in 1847, was designed to be loaded easily. The fact that it made guns much more deadly was an accidental bonus.
Muskets and barrel-loaded rifles were slow to load; traditionally, one used a ramrod and a mallet to seat the charge and the bullet firmly in place. The tight fit improved aim and muzzle velocity, both of which could be questionable in those days. In 1800 the development of the Baker rifle came in step with the innovation of using a greased patch -- reportedly the invention of the American rebels and used to great effect in the American revolution -- an easy way to close the gap in diameter between the barrel and a smaller musket ball, while additionally providing some lubricant. This decreased loading time to the point that a trained rifleman was expected to be able to fire two shots in a minute.
The Minié rifle -- of which the Minié ball was a design element -- did away with the patch and the mallet, although it still required a ramrod. This decreased loading time, allowing a rifleman to get in three shots a minute. It did this by redesigning the bullet: the 'ball' was no longer a ball, but a proper bullet shape; more importantly, the bullet had a 'skirt', a hollow segment at the rear end, that upon firing would be pushed outward to form a tight seal with the barrel. This meant that the bullet no longer had to be hammered in, becoming self-fitting to the diameter of the barrel when fired.
These bullets surprised everyone by also being much more accurate than earlier shot; a smoothbore musket was accurate at up to 200 meters, while a Minié rifle was accurate up to 550 meters, and could be used effectively to kill enemies at over 1000 meters, albeit with much less accuracy. It should be noted, however, that sources vary greatly in their estimates of range, agreeing only in that it was indeed a significant increase over previous ranges.
These rifles were originally used on people the Europeans did not care about; one of the first test cases were British soldiers killing Bantu overseas, and while they remarked on how wonderfully effective the rifle was, there was little attention paid to those wounded and killed. They were used to great effect in the Crimean War by the British, although the power of the new weapon was overshadowed by the Charge of the Light Brigade, a terrible winter, and the British officers maintaining an 18th century mindset, firing with the new rifles but still closing to fight with bayonets once the Russian lines were broken.
The American Civil War was the first Western conflict in which both side were armed with rifles firing Minié balls, and neither side was prepared. European military tactics, including those drilled by the French when training American troops during the Revolutionary War, were not yet adapted to long range rifles, guns that would allow troops to quickly advance into range that would threaten field artillery. Troops were marched into battle with the assumption that they would have be fairly close-range to engage in battle effectively, and were mowed down quickly by the surprisingly accurate and powerful guns. The wounded no longer needed doctors to dig out musket balls from their bodies; the new bullets sliced clean through the body, killing the target or shattering bones, making amputation the order of the day.
The Americans shifted to mass-production of the Minié ball and likewise into rifles, but supply lines were strained and not every soldier was able to get an appropriate rifle. It is most likely that the reports of Minié balls tumbling -- and thus doing even more damage to those hit -- were due to Minié balls being fired from older, smoothbore muskets, or improperly sized rifles. It is not inherent in their design, and would reduce their effective range.
Today, we would consider the Minié ball a fairly standard, or even sub-standard, bullet. It does nothing very special on impact, and it was set apart from earlier, round shot in that it was less likely to weave or stall in its course through the human body, with a higher velocity and more aerodynamic form punching straight through human soldiers. In 1861 this was a new and fearsome weapon, and the death toll was enormous.
Minié was orignally pronounced 'Mih-nee-ay', but the Americans turned it into 'min-nee'. Today, either is considered correct.