Receptacles -- # of wires per screw

A backfeed outlet has holes in the back. You strip the wire and "stab" it into the hole. It has spring clips inside that grab the wire.

They save a lot of time because you just strip and stab. But many professional electricians won't use the backstab method because over time the copper clips heat up and create a bad connection. Once they start heating up the extra resistance creates even more heat until the connection fails. Most common symptom of this is intermittent operation. Backstabbing has a much higher failure rate than wire wrapping around the screws.

It's a shortcut that can bite you down the road.