© Ian Graham
A slow cooker is countertop electrical appliance. It is a pot with a cover, designed to slowly cook the food put in it over a period of hours at a low temperature, in a moist environment.
Some people refer to them as “crock pots”, but a Crock-Pot® is actually a brand of slow cooker made by Rival. West Bend calls their line of slow cookers a “Slo-Cooker®.
A slow cooker comes with a ceramic or porcelain cooking pot, aka the crock. The crock will be round or oval. Some brands provide metal crocks. Standard sizes are 3 to 4 litre (3 to 4 1/2 US quarts); larger ones are 5 to 7 litres (5 to 7 US quarts). The maximum size seems to be 8 litres (8.5 US quarts.)
The crock rests inside a metal heating housing. The heating elements are generally at the bottom and / or part-way up the sides of the unit. Most have floral designs on the outside.
The assembly has a lid that covers the pot. The lid is usually see-through glass. Some have hinged lids, but the hinges can be fragile. The lids are designed so that vapour hitting the glass will condense and go back into the food.
The temperature of a slow cooker increases the longer it is on. Most dials have three settings: keep warm, low and high. The “keep warm” setting will keep a temperature of between 62 to 74 C (145 to 165 F.)
The keep warm, low and high settings are an indication of wattage being drawn on, rather than an exact temperature. The reason they can’t give you exact temperatures is because it will vary by model, how much food is in there, room temperature, etc. Food cooked in a crock will reach an internal temperature greater than 74 C (165 F) for food safety reasons. Some models may automatically switch temperatures ranges for you. For instance, some will automatically switch to “keep warm” after a set amount of time. Some are programmable, allowing you to delay the start of cooking, indicating how many hours on low and high, etc.
Very few recipes call for any stirring or anything that requires removing the lid often, because removing the lid causes a loss of heat which sets cooking back.
A slow cooker can be set to “keep warm” and put on buffet tables to use as a serving dish for foods that need to be kept warm for safety and taste reasons. Slow cookers are often carted to community meals for this reason. Many models come with bands or clamps to help secure the lid on the crock during transport.
Slow cookers are not great for reheating food, as it takes far too long, though a few models now come with oven-safe crocks and crock lids for reheating in an oven.
Some people who have a slow cooker never use them; others a lot.
There has been a general belief that slow cookers are energy efficient, but that belief has been challenged.
Slow cookers “run with lower energy wattage (70-250) than a conventional electric oven ( 2000-3000). However, while slow cookers’ heating elements stay on continuously, electric ovens cycle their elements on and off as needed to maintain temperature, often being on for only about one fourth of the actual cooking time. Assuming you would use a slow cooker on high for twice as many hours as your electric oven, the energy usage would probably come out about the same. ” 
Other factors with oven use include being able to fit many things in at once.
Much of a slow cooker’s heat is lost out through the sides, which also makes them dangerous to touch during operation.
There are now insulation slow cookers on the market. In addition to reducing their energy consumption, the insulated sides also make the sides safe to touch during operation. And, onsulated slow cookers can be good for cooking in the summer months, as the heat in them is contained: consequently, they don’t heat up the whole house, which you then have to run the air conditioner to cool back down.
Manufacturers of Oven Bags don’t recommend their use in slow cookers. They recommend instead the Oven Bags cousin, slow cooker Liners: “Reynolds® Slow Cooker Liners were specifically developed and sized for use in slow cookers, while Oven Bags were designed and sized to hold large cuts of meat for pan-roasting in the oven. There is enough of a size and shape differential between the products that we recommend using each product for its intended use.”  Oven Bags have their opening at the end; Slow Cooker Liners have their opening at the wider side. Reynold’s liners are 13” x 21”.
Slow cookers are good for cheap cuts of meat; they essentially braise or stew them.
Some say slow cookers will only produce pedestrian and mediocre meals. They say the root of the cause is that most slow cooker recipes are geared to people who can’t cook to start with — many of the recipes call for cans of soup, and there is no searing of meat, no sautéing of vegetables first, nothing that would develop flavour in a normal recipe.
The food prep for slow cooker cooking is really important for good quality cooking results, rather than just throwing everything in raw. Beef and pork pieces are often best browned before putting in. Some new models will let you brown meat in them. Any meat should be thoroughly thawed first before going in.
When converting regular recipes for slow cooker use, bear in mind you may need to reduce the liquid, as most liquid gets retained.
When doubling recipes, keep in mind that flavours intensify in a slow cooker. So instead of doubling spices, just increase them by 1 1/2 times. You may not need to completely double the liquid, try 1 1/2 times first, you can always add more later.
Put vegetables, especially hard root vegetables, on the bottom layer.
Add seafood in the last hour of cooking, unless otherwise specified. Add milk, cream and sour cream in the last 15 minutes of cooking, unless otherwise specified.
Dried beans must be boiled separately in water until soft before adding to recipes. Red kidney beans particularly, for health safety reasons, must be properly boiled separately. They have a toxin in them that a slow cooker won’t get hot enough to destroy.
Pasta is best cooked on its own in water, normally, then added in.
Long grain converted rice is the best rice to use in a slow cooker.
Don’t preheat the slow cooker with nothing in it.
If the electrical power goes out during a cooking session, try to finish cooking on the stove if you have a gas stove.
You can do steamed puddings in a slow cooker (basically you just use it as the steamer.)
To roast a chicken in a slow cooker, put the chicken in on high for about 2 1/2 hours. The secret to getting it to “roast”, and come out brown as opposed to sickly white, is to use a turkey baster to periodically suck out the juices that accumulate. This ensures the “dry heat” needed for roasting. Put the juices you suck out in a measuring jug, and use for gravy afterward, or, freeze for soup stock.
Some older crockpots from the 1970s may contain asbestos in the lining of their metal housings.
If you really want to know if your food is getting hot enough for safety: fill the crock two-thirds full with luke-warm water, set on low. After two hours, check that water temperature has reached at least 70 C (158 F.) 
The first slow cooker was introduced in 1971 by Rival as the Crock-Pot®. Crock Pot is still a trademarked name.
Rival based the Crock-Pot® on the “Naxon Beanery All-Purpose Cooker”, which Rival Industries acquired in 1970 when they bought the Naxon Utilities Corporation of Chicago.
Rival’s Crock-Pot® revolutionized cooking in the 1970s, allowing women more time away from the stove, and kitchen. They used the slogan, “Cooks all day while the cook’s away.”
Removable crocks were introduced in 1974.
After a time, slow cookers acquired a reputation for being naff, but in the recession that started in 2009, sales soared again.
In 2014, a survey by Houzz found that 44% of UK households said they used a slow cooker regularly. 
Literature & Lore
“If your oven temperature control turns the oven on for a total of 15 minutes out of every hour, your only using 1/4 of the 2400 watts the element is rated at, 600 watt hours of energy to cook for one hour. Our oven ran for 12 minutes out of every hour, this will vary greatly from oven to oven…Our slow-cooker is a large 5-quart brand with a low cooking power of 180 watts and a high cooking power of 250 watts. It’s information you find on the label. The difference between the oven and the slow-cooker is that the slow-cooker cooks continuously. What that means is if you cook on high with a 250 watt hour element for 4 hours, you use (4 X 250) 1000 watt hours of electricity. If you cook on low with a 180 watt hour element for 8 hours, you use (8 X 180) 1440 watt hours of electricity. No energy savings at my house.” — Cote, Doug. Slow Cookers and Energy Usage. Accessed January 2016 at http://www.stretcher.com/stories/01/010528j.cfm
 Gray, Sherry. University of Connecticut Extension Service. Are Crock Pots Economical? Accessed January 2016 at http://sustainableliving.uconn.edu/articles/crockpots.php
 Reynolds Kitchens: Products: FAQ. Reynolds® Slow Cooker Liners. Retrieved February 2011 from http://www.reynoldspkg.com/reynoldskitchens/en/faq_detail.asp?info_page_id=748&prod_id=3200&cat_id=1337
 Roberts, Anna. Britons are obsessed with kitchen gadgets, survey finds. London: Daily Telegraph. 27 July 2014.
 New Zealand Food Safety Authority. Cooking safely with slow cookers and crock pots. Retrieved August 2010 from http://www.nzfsa.govt.nz/consumers/food-safety-topics/food-processing-labelling/slow-cookers/index.htm
Consumer Reports. Slow cookers can save you time and money. Video. 18 February 2009. Retrieved August 2010 from http://blogs.consumerreports.org/home/2009/02/slow-cookers-crock-pot-rival-all-clad-hamilton-beach.html
Crock-Pot® Frequently Asked Questions. Rival Corporation. Retrieved June 2006 from http://www.crock-pot.ca/faqs.aspx
Fownes, Nadine. Sold on the slow cooker. Halifax, Nova Scotia, Canada: The Chronicle-Herald. 11 November 2009.
Nelson, Rick. 10 appliances that shook the world. Star Tribune. 17 April 2002.