Konbini are small convenience store that may be mistaken for Seven Elevens but they are much more! Their main characteristic is that they're always open 24/7and they are literally on every street corner in major cities, and a little more scattered in villages and rural areas. It is said that there is one for every 2000 inhabitants. The three largest distributors are Family Mart, 7 Eleven and Lawson, but there are other smaller ones such as Ministop.
The primary purpose of a konbini is selling food items of rapid consumption, which Japanese people do a lot. The most popular food you can buy there are the bento (lunch box), instant ramen (they always have hot water to eat them on the spot) and onigiri (rice balls), but they also have of course sushi, fried chicken, sandwiches, salads, and vegetables. We went there primary to buy food for breakfast (since many ryokans don't serve one - and the choice of sweets/cookies is very good) but for fast lunches as well, as many of them have also tables and stools where you can eat in. Lastly, they also sell basic bandages, medicines, and shampoo/detergents so sooner or later you will end up there during your trip to Japan!