Undermount sinks are designed so that the sink rim sits beneath, rather than on top of, your countertop. Depending on the size of the sink and the sink material, the sink may require a support frame to hold its weight, particularly when it's filled with water. The rim of the sink attaches to the bottom of the counter with special clips that hold it in place. The overhang is the place where the counter meets the sink rim.
Typically, the term "overhang" describes a small lip of countertop that extends over the edge of the sink to cover the unfinished sink rim. However, contractors may also use the term to describe any joining of the countertop to the sink edge. A positive overhang carries the countertop slightly over the edge of the sink so that the sink edge is completely obscured by the counter. Negative overhangs do not completely cover the sink rim, allowing a portion of the sink's edge to be visible beneath the counter. Flush or no-overhang designs bring the counter completely even with the sink's edge.
Positive overhangs are most commonly recommended by builders. Positive overhangs completely cover the sink installation hardware and any caulk lines. Regarded as the most structurally sound design option, positive overhangs also provide complete drainage from the counter into the sink, ensuring that no food debris or liquids become trapped in the seam between the sink and counter. If you ever need to replace your sink, the positive overhang provides some leeway in sizing, a benefit that negative overhangs and flush overhangs do not offer.
The overhang should be designed so that it provides sufficient coverage to provide aesthetic and structural benefits, but it should be small enough so that you can use the sink without obstruction from the counter. A standard positive overhang measures one-quarter of an inch over the sink rim, although contractors may create overhangs as wide as three-eighths of an inch, depending on your design preferences and the installation specifications of the sink.