In other words, why you are good for each other. In the process of achieving this, you may want to explain your interest in art history and whether you think you will pursue an academic or museum career. Be careful, however. Although you want to note your general areas of interest don't identify a specific research focus that you intend to pursue. You simply don't know enough about the field to be able to determine that at this point.
Letters of recommendation are also extremely important.
Make sure that the professors you choose to write your letters are art historians or in a related field, as distinguished and senior as possible, and familiar with your academic accomplishments. If you can ask someone to write you a letter who has contacts with the program to which you are applying, that is a plus. One final word on strategies for applying to doctoral programs. If, as a B.A., you apply directly to Ph.D. programs that indicates that you are serious about a career in the field. On the other hand, getting into the top schools is extremely difficult.