The Raspberry Pi, unlike your tablet or computer is a bare computer board - essentially like a person with the top of their head missing and the brains exposed. Sure it's fun to poke and prod, connect hardware and sensors directly to the GPIO pins on the Pi, but those have no protection circuitry on them!
Since pins are exposed its really easy for people to damage the Pi by static shock or misconnecting pins.
When using the GPIO pins, never connect the 5V or 3.3V power pins to ground, even for a moment! This will damage the regulator permanently. Also, never connect 5V or higher or less then 0V to any of the GPIO pins! This can also permanently damage the Broadcom processor