From the pamphlet, "National Honor Society," published by the National Association of Secondary School Principals:

The National Honor Society (NHS) is the nation’s premier organization established to recognize outstanding high school students. More than just an honor roll, NHS serves to honor those students who demonstrate excellence in the areas of scholarship, leadership, service, and character. These characteristics have been associated with membership in the organization since its beginning in 1921. … Today, it is estimated that more than one million students annually participate in activities of NHS and its middle level counterpart, the National Junior Honor Society (NJHS). NHS chapters are found in all 50 states, the District of Columbia, Puerto Rico, many U.S. territories, and Canada. Both NHS and NJHS were founded by and are programs of the National Association of Secondary School Principals (NASSP).

Four main purposes outlined in the NHS constitution have guided chapters from the beginning: "To create enthusiasm for scholarship, to stimulate a desire to render service, to promote leadership, and to develop character in the students of secondary schools." These purposes also translate into the criteria used for membership selection in each local chapter.

NHS is more than just academic recognition. The NHS chapter establishes guidelines for selection that are based upon a candidate’s outstanding performance in the areas of scholarship, service, leadership, and character….Specific standards for these four criteria may vary from one school to the next. Such variations are acceptable as long as they do not fall outside the parameters set by the National Council of NHS. All chapters are required to record and publish the criteria and general procedures used in their local selection processes. Final selection is made by a vote of the five-member Faculty Council, a selection committee appointed by the principal.

SOURCE:
National Association of Secondary School Principals. National Honor Society. [Reston]: NASSP, 2007. N. pag. NHS & NJHS: Official Site of the National
      Honor Society and National Junior Honor Society. Web. 2 Jan. 2012. <http://www.nhs.us/portals/3/content/55490.pdf>.