Protected 18650 Li-ion batteries have a small electronic circuit integrated into the cell packaging. This circuit protects the battery against common dangers, such as overcharge, over discharge, short circuit/over current, and temperature. Protected batteries are safer to use in your devices. Protected batteries are less like to ignite and cause personal or property damage.
Most protected batteries have these components:
PTC: Protect against over heating and indirectly over current. Will automatically reset.
CID or Pressure Valve: Will disable the cell permanently if the pressure is too high in the cell (Can be due to over charge).
PCB: Will protect against over discharge, over charge, and over current, depending on design. The PCB will reset automatically or when placed in a charger.
The following images show you examples of the components of a safe, well-designed protected 18650 battery.