In the states, we have our major convenience stores like 7-11 and Am Pm as well as smaller stores. Japan has their fair share of conbini(convenience stores). In major cities like Tokyo, they are far more saturated than what you would find here. They have 7-11 and Am Pm but they also have Lawson, Sunkus, Family Mart, Daily Yamazaki and much more. They are found at train and subway stations. This is saying alot since most of the populace commute to work through these means. Im sure you will have to pass through at least 1 of these stations when you visit. In denser areas, there can be multiple locations within a 1 block area.
The conbinis dont suffer the same stigma that afflict the same chains in the US. The prepared foods that they offer are not restaurant quality but are decent enough when you want something quick or inexpensive. They have the usual sandwiches but bentos and rice balls of various kinds are on offer as well. Some locations offer oden as well. Oden is fishcake, vegetables, eggs and other goodies simmered in dashi. Nice snack on a cold day. Unlike our domestic chains, the conbinis do not offer soda fountain to dispense drinks. All sodas are sold in individual containers located in the refrigerated cases. Oddly enough, one of the few places you will see a drink dispenser is in a family-style restaurant or all you can eat place that offer an all you can drink option.
Some conbinis offer other services like ticket purchases. If you are going to the Studio Ghibli Museum, you can purchase advace tickets at a Lawson. Another item that has a loose connection to conbinis are the clear umbrellas that dot a rainy day. These are sold fairly cheaply at all locations for those who have left their own umbrellas at home or elsewhere. Alot of times, certain chains or even branches of a chain will work in conjunction with other companies to do special promotions. Much like the fast food chains partnering with movies to extend their marketting reach through the prizes found in kid's meals. Especially with anime, shops will offer one of a kind or limited run items to draw people into the store. These tie-ins can be as rare as being offered at only 1 location due to special connection.
Not all conbinis are open 24 hours but most are. Those in the stations will close with the station unless there is an external entrace. Although the location may sell hot and cold drinks in the shop, some will still have a vending machine near the entrance. This may or may not have to do with the store not being open for 24 hours.