Kome is the word for harvested but uncooked rice - the ingredient "rice" you buy in the supermarket and store in your kitchen cabinet. The rice plants in the fields are called ine and cooked rice is gohan.
There are many varieties of rice in the world - it is the grain with the highest production for human consumption (if we include non-human consumption such as cattle feed, it would take second place after maize). The rice traditionally eaten in Japan (and the only one suitable for traditional Japanese meals) is called Oryza sativa Japonica. "Oryza sativa" is the rice grown in Asia (different from Oryza glaberrima, the rice grown in Africa). But even among Asian rice there are many varieties - the Indians for example, prefer rice that is dry and doesn't stick too much together ("like two brothers - close but not sticking too much together"), while the Japanese prefer a short-grained subspecies of which the grains stick together like they themselves in their full country.
The traditional method for cultivating rice is to flood the fields after planting the young seedlings, as this reduces the growth of weeds and deters vermin. This requires a high level of organization and cooperation among farmers - in early historical periods, this gave rise to Japanese social organization. In Japan, planting and harvesting is highly mechanized nowadays.
After harvesting, the grains are first treated with a rice huller to remove the chaff (the outer husks of the grain). The result is called "brown rice," genmai. Brown rice lacks the fragrance of hakumai and is mainly sold in organic restaurants and shops. Usually also the bran (the rest of the husk and the germ) is removed to obtain "white rice," hakumai.
A product in between brown rice and white rice is "sprouted brown rice," hatsuga genmai, which has a softer texture than brown rice, yet retains its health benefits. But this, too, is only sold in small quantities as white rice rules the day.
In Japan, the white rice meant for food is milled (also called "polished") which means that the outer layer of the grain is milled away, to obtain a finer taste. Food rice is milled down to an average of 92% of the grain. Rice as ingredient for other purposes, such as sake making, is milled much further, to 70%, 60%, 50% and even 35% of the original size - this to remove off-flavors in the sake and obtain a purer taste.
White rice is sold milled in supermarkets in plastic bags of 1, 2, 5 or 10 kilograms. In the countryside, one can find coin-operated rice polishing machines, where farmers can polish their own rice.
White rice keeps very long (it was not for nothing used as money and to pay taxes in traditional Japan), but lacks certain important nutrients. It must therefore be supplemented with other dishes such as pickles (tsukemono).
Rice bran, called nuka, is used for many purposes such as the white powder that coats Japanese sweets (wagashi) or for making one type of pickles (nukazuke).
There are two types of Japonica rice: non-glutinous (called uruchimai) and glutinous Japonica (called mochigome). The first one is used for normal cooking, the second one for special purposes such as making rice cakes (mochi), red rice (sekihan), steamed glutinous rice (okowa) and Japanese sweets.
In the last sixty years, many types of "rice brands" have been developed in Japan, just as there are different tasting brands of potatoes or grapes. Popular brands are Koshihikari, Akitakomachi, Sasanishiki, Hitomebore, etc. Most Japanese prefectures have developed their own brands. For the production of high-quality sake, there exist several brands of special "sake rice" which have larger grains and more starch - and these also are "branded," i.e. Yamada Nishiki or Gohyakumangoku, etc.