Schadenfreude is experiencing pleasure at the misfortunes of others. A rival suddenly goes bankrupt, an enemy takes a fall, karma finally catches up to a shady dealer… and you smile at the news, without a touch of consideration or remorse.
An ugly reaction, really, but it's an easy emotion to experience.
Mudita (Muditā in Pāli) is the opposite: experiencing pleasure from the well-being of others. A rival achieves a big goal, an enemy becomes a millionaire, a shady dealer wins a prestigious award… and you smile at the news, without a touch of anger or envy.
A lovely reaction, really, but it's a difficult emotion to experience.
That's because mudita is a skill, and skills only get better if you practice.