Credits (cR) are the points used to purchase armor in the The Armory. cR can be obtained through completing missions in campaign, finishing games in multiplayer, and playing firefight. It isn't yet known if custom games will result in cR.
Getting your hands on cREdit
Multiplayer games are seemingly the easiest way to get cR. Depending on your performance, the medals you get, the commendations you achieve, and the daily or weekly challanges you finish, and whether or not you win.
Commendations are measures of your playing style. There are driving commendations, headshot commendations, multikill commendations, assist commendations, and many others. By looking at someone's commendation levels, you'll be able to tell how a person plays. Reaching commendation milestones will boost your cR yield by hundreds a game.
In Reach, there will be daily and weekly challenges. They can apply to any game mode including campaign and firefight. They can give you hundreds and even thousands of credits upon completion, so it's a good idea to complete these.
It's been confirmed that getting achievements in Reach will result in bonus cR.
cR and your RankEdit
cR isn't just for money purposes. The credits you obtain are also your exp for Reach. The total cR that you've obtained will be your total exp level, meaning that it's possible to rank up from playing every game mode that results in cR. Campaign, firefight, and multiplayer will give you cR and therefore they will also give you exp. This is key, as some armor will only become visible when you become a certain rank, and Elite armor is unlocked only by ranking up.
It should be noted that spending cR will not decrease your exp, and even in losing you will gain cR.