The draft or the draught of a boat specifies how deep the sits in the water.  It is the vertical distance between the waterline and the lowest point of the boat (the keel).  Knowing the draft is crucial in calculating the clearance for your boat.

Sailing boats are often available with different keel versions specifying a different draft (draught).  A smaller draft has the advantace that less deep water becomes accessable.  This can be interesting when sailing up rivers or visiting smaller harbors.  A deeper draft on the other hand will provide better close wind performances.  Some boats try to combine the best of both worlds by having a lifting keel.  The main disadvantage of a lifting keel is the added fragility.