• The satellites (up there in the sky).
• The receiver.
• Some complicated math to figure out the position.

• The original design called for 24 satellites, but now there are more up there.
• The satellites are also known as SVs (Space Vehicles)
• Each GPS satellite transmits a signal containing three important things - 
  • The pseudorandom code allows receiver to identify the transmitting satellite
  • Almanac data provides coarse orbit information, current date, time, and a status of the satellite.
  • Ephemeris data providing very precise orbit and time information. It is only considered valid for about 30 minutes.
• Each satellite has an extremely precise atomic clock on board. This is to provide an accurate timing to receivers.

• knowing each satellite position
• knowing difference between distances to each satellite
• using good clock on the receiver side (although not as good as an atomic clock on a satellite)
• adding some error correction

we can figure out where we are. The precision of several meters can be achieved with a consumer-grade devices given clear view of the sky.

• Atmospheric effects - The signal is disturbed as it passes through the atmosphere. The GPS has a built-in model to try to compensate for this. Models of the atmosphere, accounting for the ionosphere and the troposphere layers are used to predict the delay of the signal passing through. Additionally, the satellites themselves provide updates to the basic model.
• Multipath - This is when the receiver picks up signal reflections of various terrain features, buildings and so on. Multipath effects are more severe if the receiver is stationary or moves slowly.
• Number and position of satellites - Less satellites visible means worse accuracy. Typically, no GPS fix can be acquired indoors. Additionally, there is a requirement that visible satellites should be located at wide angles to each other. If the visible satellites are aligned in a line, the error increases.