This pertains to Army divisions described in the US Army Divisions & Other Units Of Interest category.
There are three basic components to the United States Army: the Regular Army, the Reserves, and the National Guard. Just to make things interesting, during wartime when a draft is in effect, a fourth component, the Army of the United States (AUS)* comes into existence. The AUS is theoretically where National Guard units, which normally belong to the separate states, go when they are activated for Federal duty; it is also where draftees go instead of the Regular Army.
This is reflected in the numbering scheme applied to Army divisions. As a general rule, the 1st through 9th Infantry Divisions were initially the only Regular Army divisions. The 10th Mountain, 11th Airborne and all the other units through the 25th Infantry Division were originally AUS units but eventually were allotted to the Regulars. The 26th through 50th Divisions were National Guard units, and everything after that was organized as an AUS division. Obviously, some of those divisions (the 82nd and 101st Airborne) were allocated to the Regulars later, and some of them (notably the 92nd and 93rd Divisions) contained National Guard units.
*The AUS was referred to as the National Army in World War I