NAD+ is the oxidized form and NADH is the reduced form involved in electron transfer, with NAD+ typically being abundant and a good electron acceptor.

“this is a pair where nad is the oxidized form nadh is the electron holding or reduced form and it normally exists in a quite biased ratio towards a lot of nad and a small amount of nadh and the way nature works is that whenever any para chemicals is skewed in one direction it's favorable to turn the one that's abundant into the one that's less abundant and so this makes nad a decent electron acceptor”