Deep-fried, battered fish (usually cod or haddock, though with a wider selection in some areas) with rather thick chips, always made from real chunks of potato rather than thin tubes of extruded mashed potato. Fish and chips are often served with mushy peas (in England), and dressed with salt and malt vinegar (or 'Sauce' in parts of Scotland and Northern Ireland). "Proper" (authentic, for-the-masses) fish and chips can be bought only from either a backstreet "chippy" or a specialist fish and chip restaurant (the latter are mostly at the seaside, although there is a national chain, Harry Ramsden's, which does quite good fish and chips, but at "tourist prices"; Mr Ramsden's original shop, near Leeds, was a legend). However, a "proper chippy" (a backstreet "fish and chip shop", or just "chip shop") is the quintessential place to buy fish and chips. In the north you can also add mushy peas to your order. These are rarer in the south of the country. In Scotland, especially Glasgow, some fish and chip shops deep-fry almost everything they sell, including meat pies, pizzas, and even battered Mars or Snickers bars. In Northern Ireland, you can also order aPastie (not to be confused with a Cornish Pasty). This is meat minced with onions, potato and spices, which is then battered and deep fried. It can be served in a bap (a soft bread bun), on its own, or with chips. Anything served with chips in Northern Ireland and in parts of Scotland is referred to as a "supper", eg, "a fish supper" or "a pastie supper".
The best ones are specialists, serving perhaps a few alternatives such as a selection of pies or sausages. They are usually located near where people live, though some good ones, especially "sit down" chippies, can be found in town centres. They can be spotted by the illuminated sign which usually has a picture of a fish and a name: either punning and piscine, such as "Codroephenia" and "The Codfather" or proud and proprietorial, "Fred's Chippy", or even both as in "Jack's Golden Plaice". Typically a lot of people eating or waiting is an indication of good food.
A "sit down chippy" is a chip shop with a separate dining room. Whilst no real one will be exactly like this, although most elements will be present, a stereotypical sit down chippie will be brightly lit and decorated in a nautical theme with yellow or blue formica-topped tables. Typically a waitress will take your order for a Cod Meal, alternatively Haddock, Plaice or another dish, and within five minutes your meal will be served: a huge fish, a mountain of chips and mushy peas. Accompanying it, in more up-market places, will be a sachet of tartar sauce, a slice of lemon, a big plate of bread-and-butter, and a pot of tea. Some will have a separate pot of hot water, either to dilute the tea if it is too strong for your taste, or to "top-up" the tea in the pot when you have poured out your first cup. On the table will be a large shaker of salt and a bottle or plastic squeezy bottle of brown malt vinegar, which is what most British will put on their fish and chips. There may even be a tomato-shaped plastic container of ketchup or a container of brown sauce. Fish and chips bought from a pub, hotel or non-specialist restaurant bear little resemblance to those from a chippy.