The base of the dish is rice and in Spain, there are two types to choose from for maximum absorption of the stock and spice flavors. Typically a small rounded grain called Bomba is the first choice.
Originally the dish was made with rabbit, but Paella is all about variety and there can be one type used, a blend or no meat at all. The most common choices are rabbit, chicken, snails, chorizo, shrimp, mussels, clams and crab.
The vegetables included vary based on region, season and who the chef is. The most popular veggies are onions and garlic, artichoke, and will see fresh peas, red pepper or beans.
As for seasoning, the number one choice is saffron which gives the dish a smoky, earthy flavor even if not prepared over a fire and rich color. Salt, garlic, and paprika are often used as well.