A top mount sink has a lip that folds over the countertop. The sink rests on top of the cutout. An undermount sink rests below the granite/marble material so that a polished edge of granite, marble, quartz etc. is exposed above the sink. An undermount sink cutout requires more polishing and installation labor than a drop-in sink cutout, hence an undermount is more costly than a top mount.
For more information about sink options, please visit our Sinks Page.