The main difference is what holds each sink in place. Drop-in sinks rely on gravity — the rim sits along the countertop, while the faucet mounts to the top of the rim and is then fastened in place. Contrast this with undermount sinks, which have hidden rims and are secured by brackets attached to the bottom of the countertop. With an undercounter model, the faucet is installed on the countertop itself and doesn’t necessarily connect to holes in the rim of the sink.