From Halopedia, the Halo wiki
In the Halo series, a cutscene, often spelled cut scene, is an in-game cinematic movie in which the player(s) temporarily loses control over their character(s). Cutscenes, often used to show storyline events, are usually played between campaign levels. They are sometimes played at a midpoint of a level.
Cutscenes in the Halo shooter games are not pre-rendered movies; instead, they are rendered in real time, using in-game assets. Because of this, cutscenes can sometimes be altered by moving objects into the locations where the cutscenes are about to take place. In several cases, cutscenes are rendered in hidden areas that are (usually) totally inaccessible to players; the Secret Room On The Ark is an example of this. The Pan Cam in Halo 3's Theater mode can be used to "break into" some of these areas. In Halo 3: ODST, the first cutscene "Prepare to Drop" is accessible through the Firefight lobby by means of a glitch. This will allow players to free roam around the entire area where the cutscene takes place. In Halo 3's theater mode when watching the player's pre-made campaign game when a cutscene plays the viewer will not be able to watch it but simply will experience a flash and will continue from the view of the player.
Halo 2 cutscenes feature the ability where the playable characters will hold the weapon the player used before the cutscene. This is not seen in other games except for Halo 3, but this situation usually appears in cutscenes that take place in the middle of the level, notably only "Sierra 117"'s Quid Pro Quo cutscene and "Floodgate"'s Shadow of Intent cutscene will do so.
In Halo 3: ODST, scenes called "signets" by Bungie employees are introduced. Signets are cutscenes played in first person point of view. They are usually played after cutscenes and before gameplay. Bungie used signets to lead players and show what is going on. Signets are also present in Halo: Reach, albeit without the character's HUD.
In Halo: Reach, the armor permutation that the character is wearing in the campaign also shows up in cutscenes.
While most cutscenes in the Halo series are not pre-rendered, there is one cutscene in the original trilogy that is pre-rendered. This cutscene, which appears after the credits in Halo: Combat Evolved, shows 343 Guilty Spark flying around in space. This cutscene is part of the Halo credits video file which is entirely pre-rendered using Bink Video.
Halo Wars uses both pre-rendered cutscenes, which were created by Blur Studios, and in-game "vignettes". Halo 2: Anniversary would also receive similar treatment from the studio. Portions of Halo 4 are also pre-rendered, including the entire level Prologue.