Fried beef (yaki niku) and fried chicken (yaki tori) are important forms of fast food, each has its own distinctive sauce. You can get a big bowl of rice called a donburi with different kinds of meat and fish on it, a classic donburi dish is 'Oyako-don', which is chicken and an egg together on a donburi.
A traditional breakfast would be a bowl of miso soup, some rice, maybe some little silver fish, and maybe some sticky soy beans (Natto), with a cup of oriental tea (o-cha). There is a distinctive cold barley tea that is refreshing in summer, that's 'mugi-cha'. There's a really wide variety of teas as you can imagine- macha is powdered green tea prepared with a whisk, hard to remember the others.
Japan has had it's own style of curry since about the 1920s, very smooth, maybe a little spicier than a korma, and curry and rice is quite an important food.There's all sorts of fish to be had, hard to remember all the names, and it is not unknown to have a kind of fish barbecue in summer on a little brazier called a shichirin. Quite a lot of cooking 'eating out' is done in front of you, whether it's frying meat on a hot plate, cooking a shellfish until it opens, or frying an 'okonomiyaki' pancake. Okonomiyaki is wonderful, a kind of pancake with cabbage cut into it and whatever fillings you want, served with mayonaise and a date sauce.
Someone mentioned 'Shabu-Shabu', that's another good example.
Convenience stores stock a number of different snacks... 'o-don' is a kind of stew you get to pick the bits you want out of, quite a concentrated flavour I believe. Rice balls are 'o-nigiri' , glutinous rice wrapped in sea weed, and a variety of fillings to choose from. A sweeter snack is mochi, a very gluey kind of rice ball made with powdered rice and filled with sweet red beans- a very sweet dessert is uji-kin-toki, which is red beans, mochi, and ice cream together, I think it can have peanuts on it. That's in restaurants really, back in the conbini, there are all sorts of cakes to be had- 'melon-pan' or melon bread is a big cake like a half football with sugar on the top and supposedly flavoured with melon. 'An-pan' is a smaller bread, filled with red bean.
There are couple of chinese influenced traditional snacks- a 'niku-man' is steamed bread with a meaty filling, eaten with a sinus-cleansingly strong yellow mustard. 'Harumaki' are spring rolls. What else? There's quite a wide range of little condiments to put in the rice, from tiny fish to tiny crabs to different kinds of pickles, and on Kyushu 'basashi', or horse sashimi, is possible. Each area seems to have its own local dish, there's a kind of tasty rolled Tofu that you can only get in the ancient town of Nikko. I ought to mention the 'jyu-bako' as well, a special food for New Year's Day- actually, ten different dishes, each inside its own box, the whole family sits around and eats from them... I had some once, but so long ago now, I can't remember exactly what was in it... but it was delicious.... or, as the Japanese say, 'oishikatta'!
A lot of the Japanese restaurants you will see in the UK are Teppanyaki, or iron board frying- the chef has a big iron hot plate, and he fries everything in front of you. That's dento, but the staff are rarely Japanese. I suppose it's a means of cooking more than a distinct dish, they can do you okonomiyaki, or fat soba buckwheat noodles fried up, or a donburi with eel on it - unagi-don! The only time I tried this, the eel and the sauce was just right, but the rice wasn't proper Japanese rice- it should stick together. Again, each region seems to have its own rice, and the locals are very discerning over the quality of it. I suppose that's the Japanese taste, not all asians prefer rice that sticks together. Having mentioned soba noodles, I suppose it's worth mentioning the range of noodles on offer- the most famous kind is probably ramen, and instant ramen noodles are available to buy almost everywhere. You just take them home, pour in hot water, put the seasoning in, and wait three minutes, and that's all you need. Mighty... Noodles can be in a soup, fried with other ingredients, or served cold. Cold soba is another sort of fast food, just right on a hot day