Here is a schematic outline of Curiosity's MMRTG which has 4.8 kg of Pu-238 dioxide and provides 110 Watts of power (twice the maximum power of Apple Mac-book Pro)
The MMRTG from Idaho National Laboratories also provides heat to maintain a proper operating temperature of all the instruments and systems in the rover. It weighs 45 kg and is capable to produce power for almost 14 years. But, A great advantage of these thermoelectric generators is that there are no moving parts associated with them. It produces power by directly converting the heat generated by the decay into electrical energy using thermocouples at an operational efficiency of 6 to 7 % which seems too low.
The generator is fueled with a ceramic form of plutonium dioxide encased in multiple layers of protective materials including iridium capsules and high-strength graphite blocks. As the plutonium naturally decays, it gives off heat, which is circulated through the rover by heat transfer fluid plumbed throughout the system. Electric voltage is produced by using thermocouples, which exploit the temperature difference between the heat source and the cold exterior (i.e. Martian atmosphere).
As @Beckett says, Most of this efficiency loss is due to thermal conductivity with Martian atmosphere.
A nuclear power source is chosen because solar panels did not meet the full range of the mission's requirements. Only the radioisotope power system allows full-time communication with the rover during its atmospheric entry, descent and landing regardless of the landing site. Also, the nuclear-powered rover can go farther, travel to more places, last longer, and also power & heat a larger (size of a car) and more capable scientific payload compared to the solar power alternative. The radioisotope power system gives Curiosity the potential to be the longest-operating, farthest-traveling, most productive Mars surface mission in history.