A Modern Paella can consist of Chicken, Meats, Fish, and most reassuringly rice, adding sauces and vegetables are optional but are indeed better options to increase taste. However the most well known form of Paella's are the Seafood ones. These can consist of Mussels, Shrimp, Lobster, Oysters and many other forms of Seafood Delicatessen.
To make sure you are getting authentic paella, make sure you see the large classic pan toasting away at the contents of your paella, this is the reason it creates the rice dish so well, the shallow pan allows the flavors from the other foods to be infused so well with the rice giving an amazing texture.
Alegrias Spanish Tapas:
9/332 Darling St, Balmain NSW 2041
http://www.alegrias.com.au/menu
Next time you think Spanish food, think Paella, but you probably already do!
Reference -
March, Lourdes (1999), "Paella", in Davidson, Alan, The Oxford Companion to Food, Oxford: Oxford University Press, pp. 566–567, ISBN 0-19-211579-0
"An assortment of paella recipes". Spain-recipes.com. Retrieved 2016-05-19.