The nuclear power source used by Curiosity and Perseverance is the Multi-Mission Radioisotope Thermoelectric Generator, or MMRTG. It produces electricity from the heat released as plutonium-238 decays, rather than through a fission reactor or a conventional battery.
An MMRTG is particularly useful on Mars because it works day and night, through dust storms, winter, and changing seasons. Its waste heat also helps keep the rovers' electronics and instruments warm in the planet's extremely cold environment. Curiosity's MMRTG was designed to provide roughly 110 watts of electrical power at the beginning of its mission.
Perseverance uses the same basic power technology. The generator supplies steady electricity while rechargeable lithium-ion batteries handle temporary peaks, such as driving, operating the robotic arm, or using scientific instruments. The term RTG is a broader category, while MMRTG is the specific generator type carried by both rovers.