The bus and the payload
A spacecraft is usually split into a payload — the instruments or crew the mission exists for — and a bus that keeps the payload alive: structure, power, thermal control, propulsion, communications, attitude control and command and data handling.
Power and thermal
Solar arrays power most spacecraft, with batteries for eclipses. Beyond Jupiter sunlight is too weak for most designs, so missions such as Voyager and New Horizons use radioisotope thermoelectric generators. Thermal control uses insulation, radiators and heaters to keep electronics and propellant within limits.
Pointing and talking
Reaction wheels, gyroscopes and star trackers point the spacecraft precisely. High-gain antennas must point at Earth to send data; deep-space missions talk through NASA's Deep Space Network, ESA's ESTRACK and other networks of large dishes.