Understanding plumbing is a great way to resolving the problems. Many times I talk to people that get aggravated with fixing leaks under their sinkhere I take away the mystery.
The drainpipe that runs through a bathroom sink usually is fitted with a pop-up stopper that raises and lowers when you pull up or push down on a knob or handle just behind the faucet body. The knob is actually the head of a lift rod fastened to a clevis, which is a connecting bar.
The sink drain pipe is connected to the household plumbing system. It connects to the trap arm with yet another slip-nut joint. (Slip nuts are what make sink plumbing repairs so easy.) Most drainpipes for bathroom sinks are 1 1/2" in diameter, although they can be smaller or larger.
Whenever there is a leak or if you are going to replace or repair any part of the bathroom sink plumbing the shut off valve at the sink is where you'll turn off the water. Note that if you are replacing the faucet or the water supply lines it's often a good idea to also replace the shut off valves since they are likely to be the same age. Valves that sit for a long time may not turn off when you need them to so changing them out can help ensure that they will work well down the road.
A sink drain is a simple creation. First is a threaded tail piece leaving the bottom of the sink. Next comes an elbow trap -- sometimes referred to as a J-bend, which is actually two pieces that attach either to the sink tail piece or to a short slip joint extension on one end and the outside pipe on the other.
Shop our selection of Bathroom Sink, Drain Parts in the Plumbing Department at The Home Depot.