From the Embassy's website:
The Japan Exchange and Teaching Program seeks to help enhance internationalisation in Japan by promoting mutual understanding between Japan and other nations. The program aims to enhance foreign language education in Japan, and to promote international exchange at the local level through fostering ties between Japanese youth and foreign youth.
The objectives of the program are being achieved by offering JET Program participants (hereinafter, participants) the opportunity to serve in local authorities as well as public and private primary, junior high and senior high schools.
I'm going to be an ALT (Assistant Language Teacher): Participants engaged in language instruction. ALTs are placed mainly in local boards of education or publicly run primary, junior high and senior high schools. There may be a very limited number of cases in which the participants are placed in private primary, junior high and senior high schools through the prefectural offices or designated city offices.
Wish me luck!