What’s the difference?
For the better part of the 20th century, the standard was for 2-pronged outlets in every home. But by the mid to late 60s, the NEC (National Electric Code) has made the 3-pronged grounded outlets the national standard based on the National Fire Protection Association's recommendation.
The three-pronged outlets are grounded, which means if the wiring becomes damaged and too much electricity tries go through the wires the circuit will break, and the excess wiring will go through the third prong and to the ground.
Before that, the 2-pronged outlets would either blow a fuse, which was troublesome and not as reliable as the circuit breaker boxes. It still had a small risk of causing a house fire or shocking the person using the electricity. They weren't inherently dangerous but compared to the grounded outlets they are not the safest choice.
Sometimes adding a properly grounded circuit isn’t a realistic option, which is where the GFCI outlets come into play. GFCI stands for 'ground fault circuit interrupter' which is an easily installed upgrade that doesn’t necessarily require a fully grounded home. They’re set up to a circuit breaker or fuse boxes that are incredibly sensitive to shortages or surges of electricity and can cut power in a moment.
They function nearly the same as the three-pronged outlets, but the fully grounded three-pronged outlets are still superior.
An electrician can install a 3-pronged outlet with very little fuss. Installing an entirely new one requires opening the walls, but a good electrician can do that with minimal damage to a home, potentially only cutting a hole the size of the new outlet box in the walls.
Most GFCI three-pronged outlets come to electricians ready for upgrading. But the first thing the electrician will do is go to your fuse box or circuit breaker box and make sure it’s been grounded enough—if at all.
If it’s not at all grounded, it’s important for the electrician to ground it. This grounding is a bit tricky because it involves entirely re-wiring the home. It will probably end up being more expensive for the homeowner up front but will end up saving them money and giving them much more peace of mind in the long run. It will also raise the market value of a home, as it will now be up to code.
If everything is free and clear and the electrician can foresee no problems, the next thing they’ll do is turn off all the electricity in the home. Shutting off the power is important because if there is a crossed wire or something is wired in a funky way, and any of it is live, it could mean death for the electrician or homeowner.
Then the electrician will remove the old box with a screwdriver and carefully disengage the wires from it. They’ll ground the new box, and reinsert it back into the wall. After they test it and put the plastic cover back on, the job is complete.