The ranks within the Marine System are fairly straightforward, where each rank is clearly delineated.
Officers present at Marine Headquarters are considered the elite forces of the Marines, about three ranks higher in terms of power from those of the same rank at other bases. All ranks present from "Lieutenant" above are usually trained here, exceptions being those in command of other Marine bases which includes Axe-Hand Morgan and Nezumi.
This page will list the Marine ranks and the soldiers who hold the titles.
The Marines do not have a system of rank insignia per se. There is significant variation in the kind of uniform worn by officers, with many options to wear a variety of dress showcasing their own eccentric style. Marines are also allowed to wield whatever weapon they are most comfortable with, no matter how unusual it is. They are even free to fight bare-handed. However, as a general rule set down by the author, uniform guidelines are as follows:
Upon first entering the Marines, new recruits are given the standard uniform, consisting of a white, short-sleeved shirt emblazoned with the Marine emblem; a simplified seagull with the word "MARINE" underneath it, on the back; a blue neckerchief; dark blue trousers; and a baseball cap also with the word "MARINE" across the front. All officers at the rank of seaman (一等兵,Ittōhei?) and below wear this. The quasi-rank of "Zatsuyō" (雑用,Zatsuyō?) adds the kanji 雑 Zatsu ("various" or "odds and ends") to the sleeves, and to the Marine emblem on the back.
Once an officer has reached the rank of petty officer (伍長,Gochō?), he or she can choose from a number of different uniforms, including his or her personal clothes. Only those who really want to appear "sailor-like" will stay with a more traditional uniform.
Sailors at the rank of ensign (少尉,Shōi?) and above are allowed to wear the coat with "Justice" (正義,Seigi?) on the back. This coat tends to be worn like a cape, draped over the shoulders with the arms not in the sleeves. It should be noted that the sleeves of these coats are often much longer than the wearers' arms.
For the rank of lieutenant commander (少佐,Shōsa?) and above, a suit worn underneath the "Justice" coat is the preferred attire.
There are, of course, exceptions: Jango and Fullbody wear their own clothes with the "MARINE" hat, despite holding the rank of "seaman recruit" (三等兵,Santōhei?). This may have something to do with the exceptional circumstances by which both of these men were brought into their present situation, or it may simply be lenience on the part of their Rear-admiral, Hina.
"Fleet Admiral" (元帥,Gensui?) is the highest Marine rank, and is the commander of the entire organization. It is just above admiral but below the World Government Commander-in-Chief and the Five Elders. The fleet admiral and the three admirals are the only ones within the Marines capable of legally initiating a Buster Call, as well as granting World Government agents or lower ranking Marines the right to do so. The same also applies to the revocation of a Shichibukai title.
A white suit seems to be the preferred attire of the Fleet Admiral, though heavily stylized according to the person. The coat possesses pure gold epaulettes and white cuffs. Both Sengoku and Sakazuki wore this, though in the anime, Akainu's cuffs were red.
"Admiral" (大将,Taishō?) is the second highest rank in the Marines. There are only three Marines who hold this title at any given time, and they are crowned as the World Government's "Greatest Military Powers" (最高戦力,Saikō Senryoku?), as each of them possess deadly combat abilities. They are the only Marines, along with the fleet admiral, able to issue a Buster Call on any island they deem a threat.
A Marine Admiral will wear a Justice coat with golden epaulettes and cuffs corresponding to the color of their alias with the exception of Sengoku, who wear red epaulettes and the standard golden cuff. In addition, the attire of the admiral will often match the color of their alias.
"Vice Admiral" (中将,Chūjō?) is the third highest rank in the Marines. These officers can have varying duties and amounts of authority. For example, one vice admiral, John Giant, is seen to have some special authority, as he was the first in the entire Marine Headquarters to receive the report of the Buster Call and could also evidently command other vice admirals to dispatch. There are always at least five vice admirals sent with a Buster Call fleet.
The general attire of a vice admiral is that of a Marine coat with epaulets and a suit and tie worn underneath. The only vice admiral to wear an attire different from others is, again, John Giant, who wears a gold and red uniform with a strain of medals on his chest. While most vice admirals wear standard blue epaulets, Garp wears black and red, while Tsuru wears purple and gold.
Vice admirals are the Marine's strongest combat force outside of the admirals, and it would seem that most of them are swordsmen and several have been shown using Rokushiki. Many Giants who are shown to be Marines are vice admirals, a testament to their immense strength. Another attribute that is common among vice admirals is that they all know and can use at least one type of Haki, though only Garp, Vergo and Smoker have been shown using it in obvious ways.
"Rear Admiral" (少将,Shōshō?) is the fourth highest rank in the Marines. As the lowest of the Admiral ranks, they have the least amount of authority, yet they still have full command over any and all lower ranking marines. Despite being the lowest admiral rank, they are still known for being powerful fighters, and in some rare cases, rear admirals of high-enough caliber and reputation are given additional responsibility and authority, in the case of the rear admirals assigned directly to marine headquarters, with Hina as an example.
"Commodore" (准将,Junshō?) is the fifth highest rank in the Marines. Commodores have command over any marines of lower rank and are tasked with authoritative matters too important for a marine of captain rank to handle. In the case of Brannnew, commodores assigned to Headquarters are given duties or responsibilities of equal importance to their higher ranked superiors. In Brannew's case, he is responsible for the assigning of bounties to pirates and criminals, and reports directly to the fleet admiral.
"Captain" (大佐,Taisa?) is the sixth highest rank in the Marines. One of the duties that a Marine captain could have is to be the commander of one of the several Marine branches established on the islands around the world. In addition, a captain is often seen navigating the seas in order to capture pirates and keep order, in these cases, the captain is the leader of the ship, as long as there are no higher-ranking Marines on board. A Marine captain is considered a strong Marine officer whose experience and battle performance are forces to be reckoned with. Most of the several Marine captains seen in the series are powerful fighters with Devil Fruit powers, high combat skills or considerable tactical knowledge.
Interestingly, while the supposed attire of a captain is a suit under the Justice coat, very few captains follow this style of dress, instead preferring the basic uniform, such as Captains Very Good and Shu, or something unique to the individual, such as Captain T Bone.
"Ensign" (少尉,Shōi?) is the eleventh highest rank in the Marines, and the lowest and last rank within commissioned officers. This is the lowest rank given permission to wear the kanji "Justice" (正義,Seigi?) on the backs of their clothing.
"Petty Officer" (伍長,Gochō?, sometimes Petty Officer Third Class) is the fifteenth highest rank in the Marines. There are no known characters who hold this title.
Once a Marine rises to this rank, he or she is allowed to wear many different uniforms or even civil clothes. However, many petty officers prefer to appear "sailor-like" and continue wearing the traditional uniform.
"Chore Boy" (雑用,Zatsuyō?, literally meaning "Odd Jobs") is the nineteenth highest, or more specifically, the lowest rank, in the Marines. Chore boys have no official uniform besides a white Marine shirt without decoration and blue Marine trousers.
"Inspector General" (大目付,Ōmetsuke?) is a special rank which has the duty of detecting and investigating instances of maladministration, corruption, or defection in the Marine organization.  The only known Inspector General at the moment is the former Fleet Admiral Sengoku.
"Instructor" (教官,Kyōkan?) is a special rank which has the duty of training Marine recruits to become strong and rise in rank. So far, resigned Marines of high prestige are requested to remain as instructors for the future generation.