I work in a grocery store in a town/small city, there are 15 aisles so it's not the smallest, but by no means a large store. Walmart, for example, is gigantic compared to my store.
Number of employees - Management typically tries to keep the number of employees (and especially full time employees) as low as possible to keep costs down while still allowing the store to function at an acceptable level.
- Cashiers and baggers come and go; some stay for years, but most are gone in less than six months. We have half a dozen or so registers (smallish store), and only two are typically kept open at a time, with 1 or 2 baggers who may be trained to run an empty register when necessary. On holidays and other busy days, there are usually enough people there to run all the registers and a bagger for each. Cashiers and baggers typically retrieve shopping carts from the parking lot, though members of management may help if necessary.
- Stock Crew - consists of about a dozen or so at any one time. Working overnight depends on the store in question; stores with a night shift will often have two separate stock crews.
- Departments - Have crews of just a few to half a dozen or more, depending on what is needed. Meat Department has the most people, followed by the deli and produce, with the bakery bringing up the rear (Many stores in the US combine the bakery and deli into one department, drastically cutting the number and variety of bakery products available)
- Dairy/Frozen - Often combined if customer activity allows it. Just a few people in my store.
- Pharmacy - usually two workers at a time in my store, though there seem to be at least a dozen regular workers who rotate between the pharmacy and an outside drug store.
- Misc - Pricing coordinator/assistant, "floaters" (move between departments and whatever else needs to be done), etc. My store only has a few people in this section.
- Management - Store manager/Co-manager, department managers/co-managers, Front End manager (cashiers/baggers)/office personnel who are often expected to help with cashiering or other issues that may arise.
Shifts - Part time and Full Time. Most workers are part time, management and core employees (such as head cake decorator and similar) are full time. Mostly, management tries to get away with as few full time workers as possible. Shifts at my store are for part time 4 to 9 hours long (9 hour shifts include a one hour lunch you aren't paid for, others receive only a 30 minute unpaid lunch or a 15 minute break, depending on length of shift), while full time gets 9 hours including unpaid lunch. If you have to work longer, you'd better have a good reason.
Jobs - Opening shift comes in early and prepares the store/departments/etc for the day. In the bakery, for example, that means preparing the bread dough (usually laid out the night before by the closing shift) and beginning the day's baking. Some bakeries bake all day, some just bake once; it depends on the store's needs and the number of workers.
- Closing shift finishes everything up, cleans, and closes down the departments/store. Different areas require different closing procedures. The departments tend more toward cleaning everything, while the floor (the aisles/shopping area) and front end (registers/carts/etc) need to be straightened and restocked. The stock room/back room(s) also need to be tidied up, scraps of tape and cardboard and other clutter need to be cleared away, and backstock (products not yet put out on the shelves to be sold) needs to be organized and ready for the overnight/next day stock crew.
Sliding doors - exactly like ChaseJxyz said.