From Halopedia, the Halo wiki
Campaign is the single player aspect of the Halo games. In each game, the player participates in various episodes of the Halo storyline. The most recurrent character that players control is John-117, the Master Chief, although roughly half of the Halo games focus on other characters and stories. The events of the Halo campaigns focus on the confrontation between humanity and the Covenant, and also feature prominent interventions by other factions such as the Flood or the Forerunners.
Campaign levels usually consist in missions in which the player kills enemies while going through the level's areas and completing given objectives; the story is established with cutscenes and in-game dialogues. Cooperative play is available in most but not all games, but the number of players can vary. Different difficulty options (Easy, Normal, Heroic, Legendary) and gameplay modifiers (skulls, scoring) also exist. The additional Spartan Ops mode in Halo 4 is loosely based on campaign and continues the game's story.
Most Halo games are first-person shooters where the player controls a single character. The Halo Wars series are real-time strategy games, in which the player can take control of dozens of troops and units at a time. Halo: Spartan Assault and Spartan Strike are top-down isometric shooters.
Halo: Combat Evolved
In Halo: Combat Evolved, the player takes control of the Master Chief. Co-op play allows a second player (also playing the Master Chief) to join in split-screen or system link. In Halo: Combat Evolved Anniversary, the second player can join via Xbox Live. There are ten levels, which can be played on four difficulties. Anniversary also adds Terminals, Skulls, and the possibility to switch between old and new graphics.
The campaign is set in September 2552, as the United Nations Space Command is embroiled in the twenty-eighth year of a genocidal war started by the alien collective known as the Covenant. The plot revolves around the crew of the UNSC Pillar of Autumn, a ship that has escaped the catastrophic Battle of Reach, only to stumble upon Installation 04, a Halo ringworld in uncharted space. With the Covenant at their pursuit, the Master Chief gathers the human survivors and engages the enemy on the surface of Halo while trying to uncover the ring's origin and true purpose. The Covenant consider Halo as a gateway to paradise, however it is revealed that it is in fact a superweapon capable of destroying every sentient lifeform in the galaxy if activated. After the battle degenerates into a four-way conflict with the outbreak of the Flood and the Forerunner constructs, the Master Chief detonates the engines of the Autumn in order to destroy Halo and closely escapes the ringworld's destruction with his AI companion Cortana.
In Halo 2, the player alternates between the Master Chief and the Arbiter. Co-op allows a second player to join the game in split-screen or system link (or via Xbox Live in Halo 2: Anniversary). Both players control the same character around whom the level focuses (ie: two Master Chiefs, or two Arbiters). There are fifteen levels (an introductory cutscene, a tutorial and thirteen full-fledged missions), playable on four different difficulties, and there are Skulls as gameplay modifiers. Halo 2: Anniversary adds Terminals and additional Skulls along with the possibility to switch between old and new graphics.
The campaign starts in October 2552. The Master Chief has returned to Earth after the destruction of Installation 04. Meanwhile, the Covenant commander who failed to protect Halo is branded a heretic. The Covenant finally discover and attack Earth, prompting the Chief to defend the city of Mombasa, Kenya. Meanwhile, the deposed Elite becomes Arbiter and is sent by the Hierarchs to quell a rebellion against the Covenant. On Earth, the UNSC In Amber Clad, with the Chief aboard, pursues the ship of the Prophet of Regret, only to arrive to a new Halo ring, Installation 05. The Chief sets out to kill the Covenant Prophet while the Arbiter is sent to retrieve the activation index to secure Halo's activation. Both end in the clutches of the Gravemind, the Flood's central intelligence, who tasks them to stop the ring's activation. The Covenant fall into a civil war as the Hierarchs order the Brutes to betray the Elites. The Arbiter manages to defeat Tartarus, Chieftain of the Jiralhanae, by forming an alliance with the humans. Meanwhile, the Master Chief is forced to abandon Cortana in the Covenant's holy city, High Charity, as it is being infested by the Flood, and chases after the last Covenant Hierarch, the Prophet of Truth, who departs to Earth.
In Halo 3, the player controls the Master Chief. Co-op allows two players in split-screen, and up to four players in system link or Xbox Live. The second, third and fourth players respectively control the Arbiter, N'tho 'Sraom and Usze 'Taham. It is one of only two Halo games in which the co-op mode has a canonical explanation. There are ten levels playable (an introductory cutscene and nine playable missions). The player can find Terminals and Skulls, and change the game's difficulty either with the four traditional difficulties or with the Skulls. It is also the first game to introduce campaign scoring. Another feature it added was the Rally Point system which allows the player to choose where in the mission they start from with most having two or three rally points (including the option of "mission start" which is labelled as Rally Point Alpha in the MCC version of Halo 3, Halo 3 ODST and Halo 4) at important areas of a mission, such as just after a cutscene or when the player would have moved to a new area of the map, such as the cutscene after defeating 343 Guilty Spark in Halo or the area after the broken bridge in Tsavo Highway (labelled as the broken path).
The plot starts in November 2552, as the Master Chief returns to Earth to stop the Prophet of Truth. After being retrieved by the UNSC, he is sent to the besieged city of Voi to destroy the Covenant's heavy defenses. The UNSC fleet then launches an attack on Truth's Forerunner ship, but he manages to activate the Portal and departs to Installation 00, the Ark, from where all the Halo rings can be activated. Miutes after, the Flood attacks Earth but is sterilized. The UNSC and the Elites join their forces and battle the last Covenant forces on the Ark, culminating in the assassination of the Prophet of Truth. The Flood also attack the Ark via the infested High Charity. The Chief retrieves Cortana and they set out to activate a newly built Halo ring, Installation 08, to destroy the local infestation. During the escape, the UNSC Forward Unto Dawn is cut in half by the Portal, with the forward section crashing on Earth with the Arbiter and the aft section being lost in space, with the Chief and Cortana aboard. The Spartan waits in cryosleep until the UNSC find and rescue them, meanwhile the Human-Covenant War is declared over.
Halo 3: ODST
Halo: Reach also features matchmaking support for campaign where players can be automatically matched to each other.
Halo: Spartan Assault
Halo: Spartan Strike
Halo 5: Guardians
Halo Wars 2
In Halo Wars 2, player controls Captain James Cutter. Co-op allows up to two players in Xbox Live link. Player two also controls Captain Cutter as leader on original campaign. The original campaign consists of 12 Missions with Captain Cutter as leader while Extra campaign contains in total 7 missions, two mission from OPERATION: SPEARBREAKER with Major Vaughan as Leader and five mission from Awakening the Nightmare with Voridus and Pavium as Leader and can be played on four difficulties. The player can find Phoenix Logs by travelling around the map on mission and unlocking skulls by completing specific optional or bonus objectives.
The plot starts in end of March 2559 or 28 years after events on Etran Harborage, as Serina awakens entire crew of Spirit of Fire before making a final dispensation. Much to Captain Cutter and Professor Anders surprise, the ship somehow being drawn into Installation 00 through slipspace travel which their own slipspace drive was used as a makeshift bomb on Etran Harborage. Red Team was dispatched to scout the Ark after receiving UNSC distress signal but only to encounter Atriox, Leader of the Banished after retrieving the AI Isabel with Douglas-042 suffers heavy wound. Through investigation on the Ark, Anders discovers a foundry that will allow her to deploy a new Halo ring in order to raise a distress beacon where the closest original site is Reach but Cutter cannot approve that plan until Enduring Conviction, Banished's flagship is destroyed. Jerome-092 and Isabel was dispatched and manage to destroy the ship while Anders activate the ring for final preparations. UNSC and Banished contested their ground on new Installation where UNSC finally able to besiege Banished and Anders wipe them out using gravity anchors. Anders however was taken along with Installation 09 away from the Ark while UNSC and Banished continue to contest their ground on the Ark.