A Virtual Military Organization (VMO) is a dedicated hobby organization that uses simulation to model the operations of a military. Virtual Military Organizations generally have a presence on the internet, similar to real military organizations. There are some VMOs of significance currently active. The most VMOs, or MILSIM (Military Simulation) units play on casual dates. Some VMO operations are played  with thousands of participants involved at any one time. Similar to gaming clans, the key difference is that VMOs are realism groups, focusing on following real life procedures as much as possible. A quick search on google will reveal quite a few VMOs that go to varying degrees of depth to pursue their goal.
Each VMO or MILSIM unit has their own simulation set. Some will just simulate a US Army unit, or other US military branch. In some cases, you might see VMOs simulating a local army/military unit of a country other than the US. Most VMOs offer a rank system based on their nation. In some cases, VMOs may form their own custom rank system. Also, they offer courses.
Realism vs Immersion
Each VMO chooses to either follow an immersive style, or a realism, or a combination of both. While all VMOs will incorporate a rank, some go as far as implementing a Military Logistics system, including an economics model. Also, some groups go as far as making custom debriefing reports 
While each has its benefits and followers, a mix between the two seems to be the most popular option.
Some VMOs can be creative by making their own custom paint schemes, medals and award ribbons, rank insignias, and even weapons, vehicles, aircraft  and bases  (Air bases mostly developed for Flight Simulator X).
Some groups tend to form a sort of, historical, political, and battle image or reputation over time. They support various causes and styles of order/law or internal hierarchy structures. Some groups might go as far to form specific government types, ranging from Communism to Fascism, Democracy to Monarchy, and so on. At times, there are even Anarchist rebel groups, or militias that form temporarily. Groups might even grow to form into completely custom government entities or structures, and on into resembling small nations, with their own flags and codes of honor. This mostly takes place in the groups that are part of the ITTN VR-World Community, but this might occur elsewhere. The ITTN basically serves as a hub for VMOs that operate and interact politically in a Virtual Reality Political World, commonly referred to as "The VW, or THE MAP."
There are some popular games used by the VMO community including ARMA, Microsoft Flight Simulator. Falcon BMS and Digital Combat Simulator. A few units play games like Call of Duty or Battlefield, but mostly those are semi MILSIM or VMO.
It has been reported that Virtual Military groups which originated within the flight simulation genre are once thought to not be as active as previously.
Today, there are multiple groups attempting to bring back the flight simulation genre of virtual military groups/factions. The reason being, is so that multiple factions can engage over different parts of the world, using real time and weather settings, creating a realistic virtual world and possible combat scenario. This, combined with multiple factions interacting politically with trades and sometimes political disputes, using different parts of the world as their homelands, may have conflicts involving combat. Members of different groups may find their group on the brink of war with another virtual military over conflicts of interest. They may be called upon to serve in virtual combat against another group. In this, factions have their own assets, their own territory, and count possible losses when they occur. Each faction may award custom medals and awards for the service their members give. Much like a real military would. However, some groups may never enter a conflict or war, but that's not to say the group can get involved secretly, to where covert or black operations can be conducted, which would involve deep espionage, infiltration and high valued targets for pilots, special forces on the ground, and even spies infiltrating the ranks of their home factions targeted faction. Some groups develop enough and go as far as having their own custom Intelligence Agency Network. Intelligence is sometimes key in winning cold wars that may never go hot. Where black operations play a big role in the armpit sweating tank shells excitement occurs. High risk, high secrecy.
- Land Force's:
- 88th Airborne Division 2011-present, 30+ active members
- 3rd Infantry Division; 2003–present, around 40 members
- 75th Ranger Regiment; 2000 - 2010, had around 120 members
- 7th Cavalry Regiment; 2002–present, around 100 members
- 10th Special Force's Group; 2009 - 2011, around 50 members (EU split off 75th Ranger Regiment)
- Unity Security Force(USEC); 2001–present, 50+ members
- 15th MEU(SOC) Realism Unit 2007–present, 2300+ members ever; around 300+ active
- Korps Commando Troepen; 2013–present, 10 active members.
- 3rd MEU Realism Unit 2013–present, 100+ active members (BF3 Xbox) Message: ThatGuy ConDon
- Panzerbataillon 911; 2007–present
- Royal Canadian Airborne Dragoons 2013–present, 10 active members.
- Talon Republic, 30+ active members. www.talonrepublic.com
- 95th Battalion Realism Unit; 2012-present, 15 active members.
- 173rd Airborne Brigade 2012-present, 50+ active members
- 2nd Marine Expeditionary Force 2013-present, around 50 active members
- United States Special Operations Command, 2007-present, around 60+ active members
- 506th Infantry Regiment Realism Unit, 2014-present, around 180+ active members
- Air Force's:
- 476th Virtual Fighter Group; 2010–present
- VFA-113 Stingers 
- IFAF Infinite Flight Air Force ,2014-Present (100 members )  Infinite flight simulator ( A mobile sim )
- VUSN (virtual United States navy); 1999–present, around 20 members
- IFNAF ( Infinite Flight Naval Air Force ) 2014-present , 60 members
- Task Force 9 (not to be confused with IX Task Force); 2004–present, around 30 members
- IFUSN- (Ifninite Flight US Navy);was established before IFNAF mentioned above
- IFUSN BA- (Infinite Flight US Navy: Blue Angels Division; A division of IFUSN
- ITTN Factions:
- GPEO (Global Pioneers Elite Organization); 2006–present, members (site/forum 206; faction 17)
- Uatia (Kingdom of Uatia); 2007–present, members (faction 15)
- Van West, Jeff; Kevin Lane-Cummings (2007). Microsoft Flight Simulator X for Pilots. John Wiley and Sons. pp. 672–685. ISBN 0-7645-8822-2.
- Regis, Ed (2009-01-01). "Welcome to Cyberairspace". Air & Space Magazine. Retrieved 2011-03-28.
- VATSIM list of Virtual Air Forces
- Bohemia Interactive list of Virtual Land Forces
- ITTN, A site with multiple VMO's listed that interact in a separate virtual reality world, mostly on FSX and ArmA
- Official Website of the 15th MEU(SOC) Realism Unit
- Official Website of the 95th Battalion Realism Unit, ArmA II
- Official Website of the 2nd Marine Expeditionary Force, ArmA III
- Official Website of the TALON REPUBLIC, ArmA III
- Official Website of the 506th Infantry Regiment Realism Unit