There are marginal differences in combinations of food stuff and the preparations of the water, but you essentially throw a bunch of (mostly regional seafood) items into a pot to boil together.
Potato, corn on cob, and sausages can be included in the mix.
Where I'm from, seaweed & beer was used at the bottom of the pot with chicken, sausage, corn & potato before lobsters were added.
I went to college in New Orleans - crawfish were cooked in very spicy water with corn and potatoes.
Some all-you-can eat crab places use spicy, dry peppers (to get you buying more beer).....potatoes served here too.