The basis for military rank and the Chain of Command.

    Effective military units function with a clear chain of command, where the each soldier understands where they fit, and what the appropriate channels are to express concerns or complaints.  On the most basic level this is to prevent confusion about orders and infighting within a combat unit.  Skipping levels within a chain of command is a very quick way to anger both the superior who's head is being gone over, as well as the officer that the issue is being taken too.

    In order for the military to have room to promote younger members who show merit, the military generally requires that a service member leave the service if they fail to be promoted on two consecutive reviews.

Commissioned Officers

    At the top of the military hierachry (below civilian control), are officers holding a "commission" from the President of the United States to serve as his authority.  They are expected to both carry out the orders of the civilian leaders and to take a longer tactical or even strategic view of events.  Officers are considered to be of higher class than other soldiers and are thus accorded costums that we no longer see in other parts of society.  The most common example of this is the fact that they are addressed as "Sir" or "Ma'am" as appropriate (female officers will take offense at being called "Sir" and corrections may range from intentionally funny, to scathing).  Because of their leadership positions, officers are also held to a higher behavioral standard, and thus are more likely to suffer harsh consequences for what might be considered a private indescression in other cercumstances (thus the charge "Conduct Unbecoming an Officer").

Commissioned officers are divided into groups, Company Grade (because they might command a unit as large as an army company, a unit of approximately 120-200 men), Field Grade (because they might command a larger formation in the field), and Flag Grade (litterally because they have a personal flag from the days when commanders of armies in the field and large fleets needed to let the formation know where to send messages and dispatches--in most cases such flags are simple banners with depictions white stars on a red or blue background, depending on the service).

Company Grade Officers -- 2nd Lietuenants, 1st Lieutenants, Captains
Field Grade Officers -- Majors, Lt. Colonels, and Colonels
Flag Officers -- Generals and Admirals

Stargate: SG1 many of the main characters within the television show such as Colonel Jack O'Neill and Major Samantha Carter are commissioned officers, they are both considered senior officers within the SGC as Field Grade Officers.  General George Hammond is a Flag Officer.

Warrent Officers

    Between officers and englisted soldiers are specialists who hold a "warrent" from the President of the United States.  Considered officers for the most part, they are not in the chain of command or command units, and often have jobs which require specialized knowledge.  Many army helicopter pilots are warrent officers, as are criminal investigators.  A warrent officer may advance in rank without ever leaving his or her field, while commissioned officers are expected to take a number of different postings on their way up in rank.  The United States Air Force is the only branch of the military services that do not have warrent officers.  Most often referred to as "Mister" (male and female).

Non-Commissioned Officers

    The backbone of any military force are the Non-Commissioned Officers (NCOs), enlisted soldiers who hold authority without a "commission."  These are corporals, sergeants, and petty officers, and are directly responsible for the primary training and disciplining of the armed services.  The modern All Volunteer Force (AVF) has seen an end to most of the sterotypical abuse that is often depicted in films and popular culture.  NCOs are professional soldiers that large amounts of time and money have been spent on not only developing their skills as warriors, but also as managers.  But just don't call them "Sir":  they work for a living.  Enlistedmen with the rank of corporal, petty officer, or senior airmen are considered NCOs.

Enlistedmen

    Everyone else.  The average ordinary privates, airmen, and seamen.  With the introduction of the AVF in the mid-1970s most young men and women that enter the military are often seeking college benefits and intend to stay for only a short time (3-4 years), or seek to make a career and spent 20 or more years in.