Army of the Dead

They are also unique in that they are dead. This undead state apparently gave them the ability to kill with a touch and pass through enemies (again, draining the life from them) like the enemy was not even there. Meanwhile, they are the fastest of the forces involved in Minas Tirith.

Composition

Manpower

The men appear to be wraiths or green ghosts.

Weaponry

While their ghostly forms still appear to hold weaponry and armor, this seems to be more of a side effect of the fact that these soldiers are used to carrying them rather than any need for them. It must be noted that in the books they are more known for their ability to induce sheer terror in their enemies. I am unsure if the book version has the ability to kill with a touch, sucking the life from them. but the total rout of the Corsairs led to the same basic outcome. One thing was consistent though – they do not need weaponry and the enemy’s weapons could not hurt them.
This terror comes significantly from the fact that they are ghosts. It would also make sense that the life draining power applies too to help justify the fear their reputation inspires. But that reputation is also important. Many of the residents of the Southern Fiefs fled in terror once they heard the King of the Dead specifically was approaching. Another contributing factor in this was their apparent ability to switch between visible and invisible. This allows for the understandably terrifying sudden appearance of a huge ghost army right in front of the enemy.

Vehicles

They apparently fully exploit their immaterial nature. They are able to walk on water. There ones who rode horses, but those on "foot" are able to keep up with the mounted ones. At Minas Tirith, it appears they were able to out run everything present. So the 5 days it took to get from the Paths of the Dead to Pelargir says more about the limitations of the Grey Company more than the dead.

Structure

This force is also bound to Aragorn’s word given his ability to release them from the curse of Isildur. Therefore, it seems they have no independent policy and lived completely isolated “lives” where no one ever saw them. Given the feelings of shame and humiliation that they had after the curse, this makes sense as well. Thus, we can expect them to wait for Aragorn’s arrival, then to follow his every order. This fact means they are seriously underutilized in both books and movies.
The King of the Dead was the nominal head of the force. He is the guy responsible for taking the oath that led to the curse. In the movie version of events, Aragorn talked withe King of the Dead. When the reforged Narsil was able to block weapons wielded by the King of the Dead, it proved Aragorn's claim that he was the heir of isildur able to legitimately demand their obedience. Hence why that sword is important.
It appears that commands are issued via horns. When they are summoned in the books, it is with a horn. Orders to withdraw to the shore after defeating the Corsairs were issued with a horn. The Dead also acknowledged their orders by responding with their own horns.

Tactics

When one combines their speed, instant killing capabilities and ghastly appearance, you have one of the most effective fighting forces available. When deployed at Minas Tirith, they flew across the battlefield killing everything in their wake like a lethal flood. Those that resisted died and everyone else fled. Such tactics appear to be well suited for them. They give surprising and seemingly unstoppable offensive power.
But as Gimli points out, they are incredibly useful people to have around. The question becomes how best to use such a force. As you can probably expect, the movie comes the closest. Their powers are wasted unless one can do maximum damage to the enemy army. Therefore, the best application is as shock troopers. As their speed and invincibility means one does not have to worry about losses or logistics of their attack. While one does need to worry about focusing on the right point in the enemy line, with the Army of the Dead, timing is more important. No commander is stupid enough to throw their soldiers into combat against an enemy so assured of destroying the attacking army. Therefore, the Army of the Dead needs to be not only be held in reserve but kept completely secret until the enemy army is fully committed to the attack. Once this happens, the Army of the Dead can be released to completely annihilate the enemy force. Waiting longer than that and too many losses in other formations will risk the battle becoming unjustifiably costly.
In the books, they are used to defeat a proxy force that was not needed for Mordor’s victory. Thus, they were wasted. The movie moved them into the thick of battle and wiped out the entire enemy army after it had been fully committed and as soon as the Dead were available for combat. Thus the movie’s tactical application was much better. 

Their use also must be balanced at the strategic level as well and the tactical – which is where the movie fails. Their oath was to defeat Sauron totally, as was the war in which Aragorn called them to join. Minas Tirith was a defensive operation that would not have accomplished this goal no matter how victorious it was. Instead, Aragorn’s call needed to be for the duration of the war.
This means not just Minas Tirith but also the Black Gate. Here, the tactical utility of the Army of the Dead is actually reversed. You would want the enemy to know that your ultimate weapon is about to come crashing through their front door. This would serve as even more powerful bait to force Sauron to send all available troops to the Black Gate, as any reserves not used will pointlessly risk total defeat. After all, Sauron at the Black Gate was under the impression that Aragorn was in possession of the One Ring – which would make Aragorn much more powerful. Add in the Army of the Dead and any foe would need any and all power thrown into a do or die struggle for survival. The result is a pitting of all of Sauron’s remaining forces into a battle it would be doomed to lose (even without the One Ring, the Dead are too strong to be defeated in battle). This opens Mordor to a deep penetration of their territory and the destruction or conquest of all economic and military assets.
Using the Army of the Dead at the Black Gate would mean that battle would be as close to a decisive strategic level victory one could hope for short of destruction of the One Ring. Given that the Black Gate battle was effective enough of a diversion even without the added benefits of the Army of the Dead, total victory would be the result here as well.

Logistics

Recruitment

In order to summon them in the books, Aragorn went through the Paths of the Dead to reach the Stone of Erech. This is where the Men of the Mountain swore their oath to Isildur. Once he reached this location, Aragorn unfurled the banner made for him by Arwen. According to a prophecy (there seems to be a lot of those), the dead were summoned by a horn to fulfill their oaths.

History

The Army of the Dead was made up of the original people living in the White Mountains. They were related to the Dunlendings that would later cause problems for Rohan. When Gondor was founded, they swore an oath to fight for Gondor against Mordor. But they worshiped Sauron prior to the founding of Gondor. So it shouldn't have come as much of a surprise when they broke their oath to Isildur.
Isildur cursed them to never rest until they fulfill their oaths to Gondor. The result is their bodies wasting away until they became ghosts. The oath was filled when Aragorn walked through the Paths of the Dead and called on the Dead Men. In the books, they were just used to take over the Corsair ships so that Aragorn and the Grey Company could move up to Minas Tirith. All of the Corsairs fled in terror or drowned in the river. In the movies, they rode in the ships instead of the Grey Company and defeated Mordor’s armies. After these respective uses, they were released from the curse and disbanded.

Historical loyalties

The Dead Men are technically allied with the Gondorian Royal Family, of which Aragorn is the last heir of.

Overall training Level

Trained