2 Screw Light Switch. One screw terminal is for the "hot" wire that feeds the switch from the power source. Buy Screw Home Lighting Switches and get the best deals at the lowest prices on eBay!
I recently purchased a condo and am going through replacing switches and such. One black wire comes from the power source and the other goes to the light(s). Circuit electrical wiring enters the switch box.
Look for the two screws in the base of the fixture near the light sockets.
It is a little more involved than simply wiring a switch at the end or in the middle of a circuit, mainly because there are Connect the white wire from the source to the white wire or silver screw on the light fixture.
One of the black wires will bring power to the switch, the other will energize the light fixture. The circuit's bare ground wire—if there is one— is connected to the green grounding screw on the switch. If it does, turn the power back off and screw the switch in place and put the wall plate on.