To me, marinara is a simple, fresh sauce made of crushed tomatoes, onion, garlic oil olive and basil. Sometimes other herbs are used (I add oregano, for example) and usually, since tomatoes are acidic and the sauce isn't cooked for a long time, it's sweetened -- sometimes with grated carrots or as I prefer, with a sweet wine like Marsala or red vermouth.
A marinara can be used as a base for other sauces and other ingredients can be added but then it becomes something other than a marinara
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and despite the name, seafood isn't required.
Crushed tomatoes are best for texture and though other sauces, especially meat-based ones, are cooked down for a long time (hours), a marinara should take no longer than 20-30 minutes tops.