Pasta (not only spaghetti) is generally served with some sauce, as for itself it is pretty unwelcoming, as a dish. When Italians do not feel well physically, pasta is served with some olive oil and Parmigiano only. They call this kind of “sauce” “in bianco” - that is, “in white”. Lots of sauces where there is no tomato present belong to the “white” typology.
The usual spaghetti sauce you find around in the world is what Italians call “Al Pomodoro” - that is, “Tomato sauce”. Tomato sauce is quite plain - that is, featuring tomato as the main ingredient.
“Marinara” sauce is instead different. The use of the Marinara term (which means more or less “Sailor’s style”) involves the presence of garlic in the tomato. As you can imagine, when the garlic is cooked together with the tomato (some discard it before serving, while others prefer to chop it in tiny pieces and let it dissolve in the sauce), the rest of the sauce assumes a different flavor than the more subtle and plain “Al Pomodoro” sauce.
The use of the term “Marinara” - which strangely, does not have any relation with fish - is also applied to pizza, which is usually served with just tomato and garlic (no mozzarella). Quite strangely, to indicate a sauce accompanied by fish, shrimps and scallops, they use other terms - not marinara - but generally “allo scoglio” - which is “searock’s style” or “alla certosina” (reserved to risotto, usually), which is related to the Carthusian monks! As you can see, Italian is a pretty strange language, especially regarding the names of pasta recipes.
So, in the end, you are talking about ALMOST the same sauce, if you assume that spaghetti sauce is the “al pomodoro” sauce, only with the presence of garlic or not.
Always remember - do cook the pasta on the side, NOT together with the sauce! The sauce must be added only afterwards, when the pasta has been boiled and the water removed from it by the use of a colander.
You can always doctor a jarred sauce with fresh herbs, a diced onion, or other ingredients, if it isn’t exactly to your taste, but I prefer starting with a can of crushed san marzano tomatoes and adding as little as possible. Onions, olive oil, some fresh oregano, rosemary or basil, garlic and salt to taste. Maybe some red chili flakes, for the pop.