What is the difference between "ath-thawab" (recompense/reward) and "al-ajr" (wage/compensation)?

Linguistically, al-ajr (أجر) means reward. In Islamic legal terminology, it refers to what accrues to the doer, whether it is a worldly or an otherworldly recompense, as in the Almighty’s saying: "so his reward is with his Lord." As for ath-thawāb (الثواب), linguistically it means recompense, whether good or evil, though it is often used for good.