Geneva Conventions
History: The Swiss businessman Henry Dunant went to visit wounded soldiers after the Battle of Solferino in 1859. He was shocked by the lack of facilities, personnel, and medical aid available to help these soldiers. As a result, he published his book, A Memory of Solferino, in 1862, on the horrors of war. His wartime experiences inspired Dunant to propose:

A permanent relief agency for humanitarian aid in times of war
A government treaty recognizing the neutrality of the agency and allowing it to provide aid in a war zone
The former proposal led to the establishment of the Red Cross in Geneva. The latter led to the 1864 Geneva Convention, the first codified international treaty that covered the sick and wounded soldiers on the battlefield. On 22 August 1864, the Swiss government invited the governments of all European countries, as well as the United States, Brazil, and Mexico, to attend an official diplomatic conference. Sixteen countries sent a total of twenty-six delegates to Geneva. On 22 August 1864, the conference adopted the first Geneva Convention "for the Amelioration of the Condition of the Wounded in Armies in the Field". Representatives of 12 states and kingdoms signed the convention:

For both of these accomplishments, Henry Dunant became co recipient of the first Nobel Peace Prize in 1901.
On 20 October 1868 the first unsuccessful attempt to expand the 1864 treaty was undertaken. With the 'Additional Articles relating to the Condition of the Wounded in War' an attempt was initiated to clarify some rules of the 1864 convention and to extend them to maritime warfare. The Articles were signed but were only ratified by the Netherlands and the United States of America. The Netherlands later withdrew their ratification. The protection of the victims of maritime warfare would later be realized by the third Hague Convention of 1899 and the tenth Hague Convention of 1907.
In 1906 thirty-five states attended a conference convened by the Swiss government. On 6 July 1906 it resulted in the adoption of the "Convention for the Amelioration of the Condition of the Wounded and Sick in Armies in the Field", which improved and supplemented, for the first time, the 1864 convention. It remained in force until 1970 when Costa Rica acceded to the 1949 Geneva Conventions.
The 1929 conference yielded two conventions that were signed on 27 July 1929. One, the "Convention for the Amelioration of the Condition of the Wounded and Sick in Armies in the Field", was the third version to replace the original convention of 1864. The other was adopted after experiences in World War I had shown the deficiencies in the protection of prisoners of war under the Hague Conventions of 1899 and 1907. The "Convention relative to the Treatment of Prisoners of War" was not to replace these earlier conventions signed at The Hague, rather it supplemented them.
There was considerable debate over whether the Geneva Convention should prohibit indiscriminate forms of warfare, such as aerial bombings, nuclear bombings and starvation, but no agreement was reached on those forms of violence.
Inspired by the wave of humanitarian and pacifistic enthusiasm following World War II and the outrage towards the war crimes disclosed by the Nuremberg and Tokyo trials, a series of conferences were held in 1949 reaffirming, expanding and updating the prior Geneva and Hague Conventions. It yielded four distinct conventions:

The First Geneva Convention "for the Amelioration of the Condition of the Wounded and Sick in Armed Forces in the Field" was the fourth update of the original 1864 convention and replaced the 1929 convention on the same subject matter.
The Second Geneva Convention "for the Amelioration of the Condition of Wounded, Sick and Shipwrecked Members of Armed Forces at Sea" replaced the Hague Convention (X) of 1907. It was the first Geneva Convention on the protection of the victims of maritime warfare and mimicked the structure and provisions of the First Geneva Convention.
The Third Geneva Convention "relative to the Treatment of Prisoners of War" replaced the 1929 Geneva Convention that dealt with prisoners of war.
In addition to these three conventions, the conference also added a new elaborate Fourth Geneva Convention "relative to the Protection of Civilian Persons in Time of War". It was the first Geneva Convention not to deal with combatants, rather it had the protection of civilians as its subject matter. The 1899 and 1907 Hague Conventions had already contained some provisions on the protection of civilians and occupied territory. Article 154 specifically provides that the Fourth Geneva Convention is supplementary to these provisions in the Hague Conventions.

Despite the length of these documents, they were found over time to be incomplete. The nature of armed conflicts had changed with the beginning of the Cold War era, leading many to believe that the 1949 Geneva Conventions were addressing a largely extinct reality: on the one hand, most armed conflicts had become internal, or civil wars, while on the other, most wars had become increasingly asymmetric. Modern armed conflicts were inflicting an increasingly higher toll on civilians, which brought the need to provide civilian persons and objects with tangible protections in time of combat, bringing a much needed update to the Hague Conventions of 1899 and 1907. 
In light of these developments, two Protocols were adopted in 1977 that extended the terms of the 1949 Conventions with additional protections. In 2005, a third brief Protocol was added establishing an additional protective sign for medical services, the Red Crystal, as an alternative to the ubiquitous Red Cross and Red Crescent emblems, for those countries that find them objectionable.

Commentaries: The Geneva Conventions of 12 August 1949. Commentary (The Commentaries) is a series of four volumes of books published between 1952 and 1958 and containing commentaries to each of the four Geneva Conventions. The series was edited by Jean Pictet who was the vice-president of the International Committee of the Red Cross. The Commentaries are often relied upon to provide authoritative interpretation of the articles.

Contents: The Geneva Conventions are rules that apply only in times of armed conflict and seek to protect people who are not or are no longer taking part in hostilities. 
The first convention dealt with the treatment of wounded and sick armed forces in the field. The second convention dealt with the sick, wounded, and shipwrecked members of armed forces at sea. The third convention dealt with the treatment of prisoners of war during times of conflict. The fourth convention dealt with the treatment of civilians and their protection during wartime. 
Individuals who fulfill the criteria of protected persons in international armed conflicts are protected by the 1949 conventions. Those not listed as protected persons in such conflicts are instead protected by international human rights law and general treaties concerning the legal status of aliens in belligerent nations.

Conventions: In international law and diplomacy the term convention refers to an international agreement, or treaty.

The First Geneva Convention "for the Amelioration of the Condition of the Wounded and Sick in Armed Forces in the Field" (first adopted in 1864, revised in 1906, 1929 and finally 1949);
The Second Geneva Convention "for the Amelioration of the Condition of Wounded, Sick and Shipwrecked Members of Armed Forces at Sea" (first adopted in 1949, successor of the Hague Convention (X) 1907);
The Third Geneva Convention "relative to the Treatment of Prisoners of War" (first adopted in 1929, last revision in 1949);
The Fourth Geneva Convention "relative to the Protection of Civilian Persons in Time of War" (first adopted in 1949, based on parts of the Hague Convention (II) of 1899 and Hague Convention (IV) 1907).
With two Geneva Conventions revised and adopted, and the second and fourth added, in 1949 the whole set is referred to as the "Geneva Conventions of 1949" or simply the "Geneva Conventions". Usually only the Geneva Conventions of 1949 are referred to as First, Second, Third or Fourth Geneva Convention. The treaties of 1949 were ratified, in whole or with reservations, by 196 countries.

Protocols: The 1949 conventions have been modified with three amendment protocols:

Protocol I (1977) relating to the Protection of Victims of International Armed Conflicts
Protocol II (1977) relating to the Protection of Victims of Non-International Armed Conflicts
Protocol III (2005) relating to the Adoption of an Additional Distinctive Emblem

Application: The Geneva Conventions apply at times of war and armed conflict to governments who have ratified its terms. The details of applicability are spelled out in Common Articles 2 and 3.

Common Article 2 relating to international armed conflict (IAC): This article states that the Geneva Conventions apply to all the cases of international armed conflict (IAC), where at least one of the warring nations has ratified the Conventions. Primarily:

The Conventions apply to all cases of declared war between signatory nations. This is the original sense of applicability, which predates the 1949 version.
The Conventions apply to all cases of armed conflict between two or more signatory nations. This language was added in 1949 to accommodate situations that have all the characteristics of war without the existence of a formal declaration of war, such as a police action.