Board certification is the highest possible credential for a medical specialist, indicating the physician has met a certain minimum level of education, training, experience and ongoing competence. Each of the different medical specialty boards in the United States is responsible for certifying specialists in their speciality. Gregory House is board certified in diagnostic medicine.
Once a physician has completed a residency, they may represent themselves as being a "specialist" in that field. However, this is no guarantee that the physician will actually practice in that area, or has kept up their skills. A physician seeking to become board certified must demonstrate not only that they are qualified in their speciality, but that they have a certain number of minimum hours of practice in the field, that they can demonstrate competence in several core medical procedures associated with that speciality, that they keep up a minimum number of hours per year, and that they keep up with developments in their field.
A physician must actively maintain their board certified status, and can lose it for failure to keep practicing, or for failure to perform enough core procedures each year.