As basic as it is, no one seems to have the true story on marinara. If you look it up in a dozen Italian cookbooks, you're likely to find as many different explanations of what marinara is and how it's made. Two basic concepts predominate, however.
Some say that marinara is a very simple fresh tomato sauce with garlic and olive oil. Alla marinara means made in a quick and simple way, with just the few ingredients easily available to fishermen, writes Giuliano Bugialli in Bugialli on Pasta. He continues, "Some people mistakenly think the phrase means 'with seafood.'"
Other authorities, like Arthur Schwartz in Naples at Table, say the association of marinara with seafood is no mistake. The name, they contend, is derived from the Italian word mare, which means sea. According to Schwartz, "Marinara in Campania is most often a tomato-based sauce with Gaeta olives, capers, anchovies, garlic, and sometimes preserved (canned or jarred) tuna."
In America, to add to the confusion, the word marinara is used to refer to any tomato sauce made without meat. Perhaps Italian cooking authority Marcella Hazan takes the wisest approach to the issue: In an extensive section about tomato sauces in her definitive cookbook, Essentials of Classic Italian Cooking, she never once mentions the term.
Marinara sauce is seasoned, typically used as a dipping sauce. Tomato sauce is just that... tomato sauce.
Marinara technically means " of the sea". So, tomato sauce becomes Marinara when you use it on seafood.
Tomato sauce is just purred tomatoes, while marinara has a lot of other spices and maybe meat. Ok the answer below me is so wrong. First you almost never use marinara in seafood.
Marinara is more sweet and has more oregano/italian spices if i am thinking correctly.