imagine a wooden box, about four inches by five inches by two inches, hollow, with a one and a half inch soundhole on the top. you hold it in both hands like a book you are presenting to someone. this leaves your thumbs on top and relatively free. there are thin strips of steel attached to the top, which you can pluck with your thumbs to make a "bwong bwong bwong" kind of sound.

kalimba is tuned to a western octave, mbira uses a whatever tuning system is standard for the region. thumb piano is a general term.

excellent photo. the gourd around it is a resonator and is optional.