You need to installing a GFCI outlet near wet locations in bathrooms and kitchens, and as simple as it is, replacing a standard outlet with a GFCI does have a couple of wrinkles.
With a standard outlet, you may connect the black (hot) wire coming from the panel to one of the brass screws on the right-hand side of the outlet, and connect the black wire from the downstream cable to second brass screw on the right-hand side. This saves space in the box (one wire connector), though if the outlet becomes damaged, anything downstream will not work. Alternatively, you can pigtail the hot wires so that only one short length of black wire goes to the outlet. (The white [neutral] wires and bare [ground] wires must be pigtailed.)
A GFCI works differently. If you pigtail all the wires, as in the second scenario above, the GFCI will work properly, as will anything else on the circuit. However, with a GFCI, you also have the option of protecting anything downstream of the GFCI. This way, if a problem occurs on an outlet or switch down the line, the GFCI will behave exactly as if that outlet was a faulty hair-dryer plugged into it, and will shut off itself and all power downstream.
Here’s how to connect the GFCI to work that way. Connect the hot wire from the source to the brass terminal marked “Line” and the neutral wire to the silver terminal marked “Line”. (Rather than wrapping the wire around a screw or pushing it into a hole, this is usually done by both inserting it in the hole at the back and tightening the screw.)
Now, remove the tape covering the two “Load” screws, and connect the hot wire on the downstream cable to the brass screw and the neutral to the silver. If you pigtail the neutral, as you would with a standard outlet, you’ll have problems. For example, if there’s a light switch next in the circuit, turning it on will trip the GFCI. The neutral must pass through the GFCI. Now, everything downstream from the GFCI is protected.