From Halopedia, the Halo wiki
Challenges in Halo 4 are daily, weekly and monthly objectives posted in-game and on Halo Waypoint. A player is awarded a certain number of Experience Points after completing an objective. Players can only view and complete challenges while connected to Xbox LIVE.
Unlike in Halo: Reach, challenges are organized by gametype. Campaign and Spartan Ops feature two weekly challenges each. War Games features two daily challenges, two weekly challenges, and one monthly challenge.
Challenges are an excellent way for players to gain bonus XP and increase their Spartan rank. However, in Halo 4 it is much easier to reach the maximum rank than it was in Halo: Reach. Once a player has reached SR-130 they stop earning XP. As such these players receive no reward beyond personal satisfaction for completing challenges.
Campaign challenges are far less prevalent than they were in Halo: Reach. Unlike the kill and objective-based daily and weekly challenges in Reach, Halo 4 weekly campaign challenges are only completed by completing campaign levels, typically on a Heroic or higher with specified skulls activated.
In the months following Halo 4's release there was an additional category: "Waypoint Challenges". These challenges were used for the Virgin Gaming Halo 4 Infinity Challenge. The first twenty players to complete these weekly challenges won prizes as part of the contest. Once the Infinity challenge was complete, Waypoint Challenges were discontinued.