Genmai-cha is green tea (usually sencha) with roasted brown rice. The rice gives the tea a rich, warm, rather nutty flavour and blunts its astringency; it is also said to help with digestion, and genmai-cha is traditionally served after a meal in Japan. Genmai-cha is sometimes called 'popcorn tea', and the flavour is indeed a little reminiscent of popcorn, but generally genmai-cha has nothing in it except rice and tea.
Like any green tea, genmai-cha should be brewed using water which is hot but not boiling; leave the kettle to sit for a few minutes after it has come to a boil, or - better still - stop it before the bubbles get too big. Ideally, the tea should be brewed for no more than a minute or two; if you leave it too long it will eventually get bitter. Serve hot and unsweetened, without milk.
Genmai-cha is sometimes written without the hyphen, and may also be written as two or three separate words. The Korean equivalent is hyun-mi-nok-cha.
Unicode Kanji is thanks to http://www.csse.monash.edu.au/~jwb/wwwjdic.html