A sink drains by way of several components. The strainer fits into a strainer body that's inserted down through the sink hole and sealed with a bead of plumber's putty. 

Underneath are a rubber gasket and metal washer, and a large locknut or retainer tightens the body in place. A straight tailpiece mounts to the strainer body with a threaded coupling.

Slip-joint couplings connect the tailpiece, the main parts of the trap, and a short threaded nipple at a tee in the drainpipe.

At the wall or the back of the cabinet, a trim piece called an escutcheon hides the connector and the nipple. The trap, always filled with water, seals the pipe so sewer gasses won't enter the house. Waste water exits through the trap and down the vented drain pipe to the main stack.

A garbage disposal, if there is one, mounts directly to a special strainer body. The trap then connects to an outlet on the disposal. Kitchen sinks may have single or multiple bowls in a variety of shapes and sizes. They're mounted in three different ways, depending on the type: self-rimming, flush or under-mounted. 

Self-rimming sinks have a molded edge that overhangs the countertop. Flush sinks are supported by metal strips around the perimeter or are an integral part of the countertop material. Rimless or under-mount sinks are fastened or fused to the underside of the countertop.

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