From Halopedia, the Halo wiki
Challenges are in-game objectives featured in Halo: Reach, Halo 4, Halo: Spartan Assault, Halo: Spartan Strike and Halo: The Master Chief Collection. Completing a challenge's requirements awards the designated amount of credits or experience points. Challenges are one of the main methods of gaining experience points for the player to rise up in their in-game ranks. Challenges are only available while connected to Xbox LIVE.
Halo: Reach challenges are separated into campaign challenges and multiplayer challenges. There are four daily challenges and one weekly challenge after every update. By completing challenges, players gain more credits and in turn rise up in ranks.
Halo 4 challenges are separated into campaign challenges, Spartan Ops challenges, and Matchmaking War Games challenges. Campaign and Spartan Ops are weekly challenges, while War Games challenges are separated into daily challenges, weekly challenges, and a monthly challenge.
Halo: Spartan Assault
Halo: Spartan Assault Assault Ops (known in the Windows versions as "challenges") are only present in the campaign. There are three weekly challenges, and 90 permanent mission-specific challenges (three per mission). Completing challenges earns players XP.
Halo: Spartan Strike
Halo: Spartan Strike Assault Ops are again present in the campaign. There are three weekly Assault Ops and 90 mission-specific Assault Ops (three per mission). Completing challenges earns players cR.
Halo: The Master Chief Collection
With the release of Halo 3 on Windows and Steam, the Master Chief Collection has received a suite of PvE and PvP challenges to allow players to earn season points, XP and armor permutations. Some challenges are refreshed weekly and others per-season.