The modern satellite is an extremely complicated piece of equipment composed of more than a half-dozen major subsystems and thousands of parts. Satellites live and die in space and are subjected to an extremly hostile environment. Below is the 'short list' of what goes into building a satellite.
COMMON SATELLITE SUBSYSTEMS
Propulsion
The propulsion system is only partly the components that get the satellite into orbit. Other chemical or electrical motors are used to move the satellite back into the correct orbit when either atmospheric drag, magnetic fields or the solar winds deflect the satellite out of it's correct trajectory. These motors boost it back to the correct altitude, speed the satellite up, slow it down, or change the angle of the trajectory.
Power
Solar panels are used in combination with batteries to provide a constant source of electrical power on the satellite. The batteries are used when the satellite is not in direct sunlight, and allows the satellite to continue to function. LEO satellites have greater need of batteries, while GEO satellites have less, as they are exposed to the sun longer.
Communications
The communications subsystem uses transmitters, receivers or transponders (transmitter and receiver in one component). The communications subsystem handles all transmit and receive communications functions. If it is a communications satellite, this will be a heavy portion of the satellite's construction.
Superstructure
The satellite must survive the violent forces of the rocket ride into space. The superstructure of the satellite not only supports it in space, but reduces the shock and vibration the internal components might suffer during the launch.
Thermal
The whole point of the thermal system is to regulate the temperature of the satellite's components. Too hot or too cold, or too great a swing in temperature will prematurely end the useful life of a satellite. This system dissipates the heat away from earth, out into space, so as not to interfere with the satellite's operation.
Attitude
The satellite must face the earth at all times. The attitude control system allows the satellite to remain pointed correctly. These are often very small motors compared to the propulsion system.
Telemetry and Command
The satellite must inform the sattellite operations center what it's current state is, and where it is located in orbit. Often a simple 'beacon' system is used to allow the ground station to track the satellite in orbit. Additional information is relayed to the ground, such as the craft's operating temperature, state of it's programs and operating system, as well as a host of other internal functions.
SUMMARY
Satellites must cope with the high temperatures of sitting in direct sunlight, and the near absolute-zero temperatures they drop down to in the shadow of the earth. The very fact that they move rapidly between the two extremes in temperature means their lifespans are very short. As if this were not enough, they must cope with the solar wind, which creates a buildup of static electricity on the satellite.
All these factors contribute to requiring that the components of the satellite be very durable and therefore very, very expensive. This is why very few companies and governments operate satellites, and only a handful of companies build them.