Military Ranks
US Military Rank Insignia Guide

US Military Ranks and Military Rank Insignia Explained

Military Ranks are more than just who salutes whom.
Military rank is a badge of leadership.
Responsibility for personnel, equipment, and mission grows with each increase in rank.

Do not confuse rank with pay grades, such as E-1, W-2 and O-5.
Pay grades are administrative classifications used primarily to standardize compensation across the services.
The "E" in E-1 stands for "enlisted" while the "1" indicates the pay grade for that position.
The other pay categories are "W" for warrant officers and "O" for commissioned officers.
Some enlisted pay grades have two ranks.

The Army, for example, has the ranks of corporal and specialist at the pay grade of E-4.
A corporal is expected to fill a leadership role and has a higher rank than a specialist
even though both receive the same amount of pay.
In the Marine Corps, master gunnery sergeants and sergeant majors are E-9s,
but the sergeant major has the higher military rank .

Source: Defenselink.com

Interesting military ranks tidbits:

Ensigns started with the Army but ended with the Navy. The
military rank of Army ensign was long gone by the time the rank of Navy
ensign was established in 1862. Ensigns received gold bars in
1922, some five years after equivalent Army second lieutenants
received theirs.

"Lieutenant" comes from the French "lieu" meaning "place" and
"tenant" meaning "holding." Literally, lieutenants are place
holders.

While majors outrank lieutenants, lieutenant generals outrank
major generals. This comes from British tradition: Generals were
appointed for campaigns and often called "captain generals."
Their assistants were, naturally, "lieutenant generals." At the
same time, the chief administrative officer was the "sergeant
major general." Somewhere along the way, "sergeant" was dropped.

Gold is worth more than silver, but silver outranks gold. This
is because the Army decreed in 1832 that infantry colonels would
wear gold eagles on an epaulette of silver and all other
colonels would wear silver eagles on gold. When majors and
lieutenant colonels received the leaves, this tradition could
not continue. So silver leaves represented lieutenant colonels
and gold, majors. The case of lieutenants is different: First
lieutenants had been wearing silver bars for 80 years before
second lieutenants had any bars at all.

Colonel is pronounced "kernal" because the British adopted the
French spelling "colonel" but Spanish pronunciation "coronel"
and then corrupted the pronunciation.

While military rank insignia are important, sometimes it isn't smart to
wear them. When the rifled musket made its appearance in the
Civil War, sharpshooters looked for officers. Officers soon
learned to take off their military rank insignia as they approached the
battle line.