9mm (or nine-millimetre, depending on your locale) is a general term for a family of firearm ammunition. The value refers to the diameter of the bullet itself; the casing may vary in size. Typically, this caliber (for you Americans) ammunition is used in handguns and submachine guns (machine pistols).
There are a variety of common subtypes of 9mm ammunition. Some include:
The most famous early adopter of the 9mm standard was Georg Luger, in the magnificent pistol that bears his name. The Luger was originally a 7.62 and 7.65mm weapon (it also even sported the .45 ACP for a U.S. procurement trial series that resulted in the U.S. purchasing the Browning M1911). In 1904, while still attempting to garner large-scale contracts, Luger retooled the gun to fire a larger bullet in order to allay concerns over the stopping power of the weapon. He designed a cylindrical, round-nosed shell, 9mm in diameter and 19mm in length. This version of the weapon was introduced in 1903/1904, and the German Army and Navy accepted it as a standard firearm (the Luger P-08, for pistole - eight rounds, its capacity) and thus began the saga of the Luger.
We are, of course, not too concerned with the Luger other than as a booster and impetus for the 9mm caliber of ammunition. During the interwar period, John Browning (creator of the M1911 for the U.S. Government) had also worked with Fabrique Nationale of Belgium to produce a 9mm handgun. Loading in the Browning 'tipping barrel' style as opposed to the more complex and expensive Luger 'knee toggle', the FN Browning Hi-Power (translated from the French original, FN Browning Grande Puissance) was manufactured before World War II until the German Army captured Belgium and, perforce, the FN works. They continued producing the FN-GP for the German military during the war, continuing on to produce it for general and military use by the NATO allies afterwards. Tooled for the now-venerable Luger 9mm, it served to even further spread the use of this cartridge type.
note: I had stated that the Walther P.38 was an update of the Luger; JesseL has corrected me, informing me that they were in fact separate designs.
Not to be outdone, the Italians chipped in through the efforts of Signior Beretta, producing a whole line of 9mm handguns. Recently, the Beretta M9 has become the standard issue sidearm of the U.S. military, at last supplanting Browning's .45 ACP.
Around this time, the German military was looking for a means to offer their paratroopers and other infantry increased firepower. They adopted a weapon known as the M-38, for machinenpistole or machine pistol, 1938 edition. This gun was a descendent of the first automatic hand weapon, the M-18, which had seen action in the First World War; it was, however, better designed, quite rugged, and fired 9mm ammunition. In addition to being a happy medium between round power and round weight (determining, after all, how many rounds a soldier can carry) the 9mm was also readily available, being manufactured in mass lots for the now-ubiquitous Luger. The weapon went through a revision to become the MP-40; in this guise, it served the Germans (and their enemies as well; the MP-40 was prized by Allied soldiers over their own nations' automatic weapons such as the .45 Thompson or the Sten gun) for the duration of the war. Afterwards, it went on to be the sire of most modern assault rifles, especially what are arguably the best known of the lot: the AK-47 and its descendents.
The Karl Walther Waffenfabrik had (after the war) begun offering other weapons that accepted 9mm rounds, such as the Walther PPK. The Soviets produced a weapon derived from the PPK known as the Makarov 59, which accepted a slightly modified version of the 9mm round - it was 9x18mm, and had a diameter of 0.365". This round, which serves Soviet/Russian arms still, is known as the 9mm Makarov and is in production today not only by some firms but by enthusiastic handloaders around the world who have found that it's much cheaper to simply mill down a 9mm Luger round.
At this point, the 9mm handgun and 9mm submachine gun became almost a de facto standard. The ammunition was widely manufactured, and relatively cheap; the round had adequate stopping power (especially in automatic weapons) for close-in work. Loaded for pistol use, it lacked range; however, the machine pistol lacks accuracy at medium or long ranges anyway, so that didn't matter. Many well-known weapons of both types are 9mm, including (a small sample):
There has for many years been a war bubbling amongst gun users and enthusiasts over the relative merits of various types of ammunition. Typically, the .45 ACP
and the 9mm
family are compared. In general, the 9mm is a smaller cartridge, weighing less; it travels at higher velocities and is more aerodynamic. Parabellum
cartridges are fully jacketed
as per international agreement; there is an enormous aftermarket
in 'specialty' ammunition ranging from wax and dye 'practice' rounds to frangible
bullets such as the Glaser Safety Slug
rounds for SMGs.