Paella is a one-dish Spanish meal based on rice and cooked uncovered in a special paella pan. It may be served with a salad.
Though made throughout Spain now, and regarded by outsiders as practically the national dish of Spain, it is a regional dish with strong roots in the Valencia region. In Spain, it is eaten at lunchtime, particularly on Sundays or festive days: only the tourists have it in the evening.
Cooks outside Spain have come to focus on the seafood in Paella, but in fact, the rice in it is the single most important ingredient. It must even be a certain type of rice. See separate entry on Paella Rice.
The main spice in Paella is usually saffron.
There are endless debates, inside Spain and outside, over the right ingredients for Paella. It is probably best right at the outset to state that there never can be a clear winner in these debates because at its most traditional, Paella draws on what you have to hand. It was originated by peasants and field-workers. Those living right on the coast would use the seafood that they could easily get for free; those more inland would use fowl such as duck or chicken, and / or rabbit.
Some very-experienced Paella cooks say that getting the technique right is more important than worrying about strictly adhering to fixed ingredients.
Some defined types have arisen, however.
- Paella valenciana: This is more of an inland, garden version. Ingredients include rice, saffron, chicken, either rabbit or lean pork, land snails, vegetables and beans. The beans might be green beans, lima beans, or fava beans, with artichokes being used in the winter when green beans are not available.
- Paella alicantina: More of a coastal version. Ingredients include rice, saffron, lobster, shellfish, shrimp and prawns
- Paella a la marinera (aka paella de mariscos): Another coastal version. Rice, saffron, crayfish, monkfish, mussels, shrimp with peppers and tomatoes
- Paella de conejo: with rabbit
- Paella de pollo: with chicken
- Paella mixta: mixed meat and seafood
- Fideuàis: a Paella that uses pasta instead of rice
Outside these definitions, some say a good Paella must have chorizo de Bilbao. Other cooks in Spain say it must have green peas, particularly for the colour contrast. (Note: this isn't done inside Valencia itself.) 
Some cooks prepare a stock to cook the rice in. But many Spanish cooks just start with plain water. For them, the seafood and the meat are the flavouring ingredients that will flavour the water, anyway. And while North Americans might fuss about the cooking of the seafood in the Paella, many Spanish cooks don't worry about that too much, regarding the seafood as just there as a flavouring agent. "Never lose sight of your goal: paella is a rice dish. It doesn't matter if you overcook the seafood, it's there mainly to flavor the rice. In Spain, they push aside the meat and seafood and just eat the rice." 
The traditional cooking fuel is wood (particularly prunings from orange and olive trees) and dried prunings from vines. Extreme purists say the wood-fire cooking is an essential part of Paella.
Simpler versions of Paella are called "un arroz" ("a rice"). These recipes are mostly rice with very few ingredients. They are used as a simple starter for everyday meals, while Paella is meant to be an impressive, dramatic meal in itself. Women usually make "arroz" dishes, while men elbow them out of the way to make (and claim the credit for) the special occasions that call for Paella.
You can get special Paella Pans, but most amateur home cooks end up using the broadest frying pan they can lay their hands on, instead. Paella purists say the pan should be broad enough to allow the rice to be no deeper than the side of a finger (i.e. 1/2 inch / 1 cm.)
Don't rinse the rice before starting; you actually want to retain any starch that might be on it.
For the basic rice component, allow per person, 1/2 cup (3 1/2 oz / 100g) of uncooked rice, and 1 cup (8 oz / 250 ml) of liquid. [If the liquid cooks off too quickly on you, you may need to add a tidge more liquid towards the end.]
For other ingredients, remember Paella is a rice dish: the other ingredients are meant to be flavouring -- per 1/2 cup (3 1/2 oz / 100g) of uncooked rice, no more than 5 - 8 oz ( 150 to 225g) of total meat (fowl, seafood, fish or whatever) combined.
The success of a Paella dish is ultimately in the taste and texture of the rice.
The pan is heated with olive oil in it. Meats are browned and veggies sautéed in this oil. The meat needs to be cut into small pieces so that it cooks quickly.
This initial fried mixture is called the "sofrito"; many say a secret to a good paella is the flavour that is developed at this initial stage. Then the heat is cranked up, and the cooking liquid is added and brought to a boil. If large pieces of meat such as chicken legs are being used, the boiling is allowed to continue for several minutes to help ensure the meat gets cooked.
Otherwise, the rice is added straightway, stirred to mix everything evenly, and the water is reduced to a simmer. Most cooks agree that you should not stir a Paella again after the initial stir because stirring frees up more starch than you want, and can make the Paella gummy. Additionally, leaving the Paella to its own devices at this stage allows a crusty layer called the "socorrat" to form on the bottom of the pan. Most people want this. You may find people who don't want the crust, but who are still reluctant to stir, shaking the pan instead.
Getting a good bottom crust can be tricky: you don't want to wait so long that the rice burns. You have to pay attention for the smell of rice toasting.
Some versions of Paella (such as those made south-west of Valencia) will stir-fry the rice in oil first.
Some recipes will call for large pieces of chicken. These tend to have you do the sofrito, brown the meat, then add water and bring to a boil, and boil a bit to cook the meat before adding the rice.
Fava beans and artichokes are often par-boiled first before being added.
Some people say that they like to finish Paella off in the oven, but don't worry about this -- it's not traditional and most Paella pans wouldn't fit in ovens, anyway.
The sofrito stage of making Paella should take about 20 to 30 minutes. Then about 10 minutes for the stage where you add liquid and bring it to a boil. Then allow about 20 minutes of simmering with the rice. The rice is cooked when it is al dente. Then, finally, a Paella needs rest time off the heat of 5 to 10 minutes before being served.
To serve, put hot mats on the table, and place the Paella pan on them to serve it. People help themselves.
Stove-Top PaellaIn Spain, Paella is made outdoors in extremely wide frying pans, over a wood fire or on a specially designed gas ring. Restaurants will have special stove-tops that can accommodate the size of the pans indoors.
At home on a regular stove top, it can be tricky to fit something as big as a real Paella pan. If you are using one, you will probably need to have your Paella pan straddle two burners. If you don't feel you have the proper heat source for even heat distribution, some say to stir the rice two or three times during the first 10 minutes after it has been added, then to leave it alone. To get the crust when cooking indoors on a stove, crank up the heat during the last minutes of cooking.
If you don't have a Paella pan, you may wish to use two frying pans, or a wok with a relatively flat bottom.
The first reference to "paella valenciana" appears to be 1840 in a Valencian newspaper.
The paddies in which the rice was grown are not near the sea (on foot, anyway), so seafood was a later addition. You would have been more likely to encounter eel, frogs, land snails, and rata de marjal (marsh rat) as the ingredients.
Peppers and tomatoes are a later addition as well, being foods from the New World which required a long time to be accepted even after they were known.