Since the early 1900s the U.S. military has standardized on a manageable number of types of small arms ammunition. These Olive-Drab.com pages will provide basic information about each major type. Although other ammunition has been used by the U.S. military (e.g. .22 cal., 12 ga. shotshells) the cartridges covered here represent by far the most common in actual usage.
Inspecting ammo at Lake City Army Ammunition Plant, Independence, MO.
Ammo cans are piled high in the background.
There are many fine websites that have additional information on this
topic, too many to list here and too many to keep up with as they come and go.
Use this Google web search form to get an up to date report of what's out there.
For good results, try entering this: ammunition. Then click the Search button.