In the Army, there is this confusing rank called Warrant Officer which appears to be below 2nd Lieutenant in rank…Is Warrant Officer something ‘between’ officers and enlisted men, or is it a CO rank? 
I’ve heard that the Army has some Warrant Officer pilots, which would make them the lowest-ranking soldier to pilot an airplane. Is this true?
What exactly do Warrant Officers do?
While I am ex-airforce and not entirely clear on this myself ( Airforce doesn’t have WOs), I can clear up some of this. I had a short conversation once with a warrant officer, sadly I only vaguely remember part of it.
Basically, yes, it is somewhere between officer and enlisted. Enlisted salute them, they salute officers as a 2nd Lt I think. There are 4 grades of WO.
I think they can be pilots, but only on certain aircraft( not sure on this at all). The one I talked to was some sort of navigator, ( or was it pilot?) /maintenance man. He got to help fly the plane, then help fix it when it landed broke.
From what I understood, they are sort of a relic, and are slowly being phased out.
Assuredly someone from one of the branches who actaully has WOs can clear this up.
From the Royal Naval historical point of view, a warrant officer was an officer by virtue of a warrant from the Navy Board, as opposed to a commision from the Admiralty.
In the Australian Defence Force warrant officers are “enlisted”; however they are addressed as “sir” by junior ranks (although not saluted).
A warrent officer brings the skills he needs with him. He doesn’t need the military job training.
Cite?
This website has a brief history of warrant officers along with a link to another more detailed page on the history of warrant officers in the navy.
Billdo, great cite.
In the Navy, Warrant Officers are people promoted from the enlisted ranks. They’ve got to have at least 14 years active service, and have to be a chief petty officer or higher (selectees to Chief are included in this group.)
A few years ago the W-5 paygrade was created, because in the early '90s the W-1 grade was abolished. Hey, it doesn’t have to be a good plan, it just has to be a plan.
in the Army, WO’s are usually in very specific technical positions such as helicopter pilots, senior maintenance folks or even reactor operators. They are addressed as Mr so and so, as opposed to Sir for an officer. They hold a warrant, not a commission as an officer. They are given plenty of respect by officers, although they are technically below even a second lieutenant in rank.
A friend of mine was a Black Hawk pilot in the 101st Airborne in Gulf War 1. I don’t know if she was addressed as “Mr.” though. 
In the Army (and I’m not sure if in the Marines), there is still the W-1 grade, and both the Army and Marines promoted people to W-5 a few years before the Navy did. Army Aviation WOs (pilots) can enter as W-1, i.e. you can enlist specifically to go into flight school as Warrant Officer Candidate, and be appointed a WO1 at graduation. Army technical WOs are selected from the NCO grades and IIRC do not require quite as long a time-in-service as Navy WOs.
Originally, the legal distinction was that WOs held their rank, appointment, etc. though a “warrant” issued by the Navy Secretary or War(Army) Secretary, as opposed to a commission from the President. Today, everyone at CW2 and above has a commission; the Army’s WO1s still have the Service Warrant, but promotion to CW2 (and thus commission) is virtually guaranteed after satisfactory completion of a specified time-in-service.
Reeder’s definition of the WO was the definition at the time the US Navy adopted the concept in the 19th century.
In the 1950s the Air Force stopped issuing Warrants, deciding instead to split the functions that used to be given to WOs between the top 2 enlisted grades and the technical-branch commissioned officers.
(U.S. Army) WO’s are specialists. They come in five ranks and have the endearing custom of not having any rank amongst themselves. (That is they first-name each other.)
They are addressed as ‘Chief’ by everyone. (Try translating that into Spanish!)
Most Army helicopter pilots are WOs, so you find them in large herds in aviation outfits. There are other, including the powerful, mysterious and much-feared Finance and Personnel WOs. Cross them and they will cancel your birthday.
An (almost) unique WO slot is as the commander of Army bands. Until recently, they were the only WOs who commanded. WOs also boss the large number of Army boats (including hovercraft).
All in all, neat people. Any other questions?
They are refered to as Mr or Ms in situations where you would use Lieutenant or Captain in addressing a commissioned officer. If its a situation where the name “LT” could be used for a LT then Chief can be used. For an enlisted soldier talking to a WO, sir in also appropriate. Navy and Marines call them “Gunner”. I have no idea why.
And yes I do mean Gunner. Gunny is different and it refers to a Marine E7 not a WO.
To answer the OP as to what they do I’ll give it a shot. In the Army there are too basic reasons for them. First I will leave out the pilots. Most of the WOs are former NCOs with a lot of experience. It is a way to get a subject matter expert some more authority and recoginition. It is also a way to keep experienced officers in important roles. For instance an experienced motor SGT can get selected for WO and after he can be a Battalion maintenance officer. They are usually found in support roles such as finance, transportation or maintenance. They are also quite a few in military intelligence. Although they sometimes have a large staff of people working under them, they are seldom in command of a unit. There is a distiction. A BMO might have 100 people working for him but he will not be a commander of a 100 man company. Its no longer prohibited but it doesn’t happen very often. Also commissioned officer must promote or perish. They have to move up the ranks and through various jobs or else they get kicked out.
For pilots its a practical reason. The Army needs a lot of pilots but not too many officers. Say there are 34 pilots in a company. One is the company commander, CPT. Two are platoon leaders, 2LT. One is the executive officer, 1LT. The rest are pilots. If they weren’t WOs you would have 30 officers trying to get to those 4 slots. They would need to get promoted or get kicked out. After the rank of CPT most pilots don’t get to do much flying. They become staff weenies and just get their minimums. /the Army would rather have those 30 pilots just concentrate on flying and not worry about which staff position they need to get promoted.
As usual I probably just made things more confused. I assure you it was all clear in my head.
Actually, when a warrant officer becomes a CW2, he receives a commission and is a commissioned warrant officer.
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www.rankinsignia.info
Is a Warrant Officer similar to a “Specialist”?
No, not even close.
A Specialist is an E-4, the same pay grade as a corporal. The term is a misnomer. They are not specialists in anything. In the same way and RAF Flying Officer may or may not fly.
I believe that a specialist in the U.S. military is any soldier who’s been promoted past private but hasn’t actually been given command over anything. Apparently you can’t be, say, a corporal unless you’re commanding a squad, because then you’d have a corporal commanding corporals and the who natural order of things would collapse.
Rank, either enlisted or officer, doesn’t really mean much in the military, beyond pay or bragging rights - your job does. The only real authority is positional authority.