KM are often refereed to as "clicks" Why? I really don't know, they have been called that long before I went into the Corps and will be called that after I am gone. The clicks on your scope depend on how your sight is built, normally that are in 1/2, 1/3, 1/4, or 1/8 or .86 MOAs. So for each click (in a perfect world) you move the bullet approx. either 1/2, 1/3, 1/4, 1/8 or.86th of an inch for every 100 yards the target is away from the sight.
A little off topic but very similar and is often asked.
Some of the military and "military want to be" sights also have reticules (mil-dot sight) that are in mils (there are some produced by the former Soviets and their allies are in DC). A mil is an angle, there are 6400 in a complete circle (the Russian DC system uses an angle that there are 6000 in a complete circle). The mil was originally a unit of measurement used by artillerymen, but later adopted by almost the entire military for instances were degrees are not accurate enough. The reason that the mil was incorporated into military sniper sights (and most optical direct fire weapons sights) are two fold, the first being the Marine officers that help develop the sight were artillery officers that were on a B-billet at the WTBn, Quantico, VA (the Bn runs the scout-sniper school). More important a mil subtends approximately 1 unit of measurement (works with yards, feet, meters, inches, etc.) for every thousand units of measure away from the measuring instrument. So if you know the size of an object, you can determine range by plugging in the mils measured into the mil relation formula or W=RxM. (W=width, height, etc. R= range divided by 1000, M equals mils measured) You can also work the formula backwards, i.e. you know how far the target is, you can determine the space between mils and use that for target lead and hold over so you don't have to "click-in your come-ups." now these are approximate measurements, because the mil relation formula is obtaining an angular measurement, this is close to but not the exact the same as the strait line distance between the two measured points on the curve. In artillery FDC it is normally to divide by 1.0186 to convert the outside angle into a strait line distance when subtending an angle.