Both challenge systems are great sources of Player experience points (XP): Daily Challenges give thousands of XP every 24 hours, and Career Challenges set up milestones to work toward throughout your experiences across Modern Warfare III, Zombies, and Call of Duty®: Warzone™ (at Season 1), granting XP and Calling Cards upon completion.
In addition to Daily Challenges, expect an assortment of Multiplayer and Modern Warfare Zombies Career Challenges at launch.
Some of these, such as match wins and kills, will come naturally by simply playing the game. Others, such as earning kills with a specific weapon category or the use of Equipment may involve changing your playstyle.
Career Challenges have several milestone steps, which award XP. You can even continue pushing to fully complete a Career Challenge, which offers additional XP and a Calling Card.
Once Season 1 (and future seasons) become available, expect additional challenges unlocked at Prestige Level milestones that offer more difficult objectives and XP rewards. Expect intel on the Prestige system closer to Season 1.