Paella and seafood casserole
Paella is a specific type of rice dish, cooked in a wide, flat pan.  Although sesfood is typical, it can also be made without seafood.  (I typically just do chicken and sausage)

Casseroles, however, are typically a higher sided earthware or similar dish, and baked.  They typically have starch, but it might be rice, noodles, or a biscuit top.  They might use a cream based sauce, or cheese, which would not be included in a paella.