In my experience marinara sauce means a thinner liquid and chunks of tomato while a tomato sauce seems to be a thicker puree. But this may not be a hard and fast rule.
Interesting question! I thought about for a minute and every marinara sauce I've had has been made with chunks of veggies in it, so that was going to be my guess - and then I checked wikipedia:
"Marinara (mariner's) sauce is a southern Italian tomato sauce usually made with tomatoes, garlic, herbs (such as basil), and onion. However, there are many variations. Some of these include the addition of capers, olives and spices.
Italians refer to marinara only in association with other recipes. For instance, spaghetti alla marinara literally translates to mariner's spaghetti. However, tomato sauce alone in Italy is called salsa al pomodoro or pummarola." http://en.wikipedia.org/wiki/Marinara_sauce
So it seems like tomato sauce is the general category, and marinara sauce is a specific type, although it seems like a pretty standard tomato sauce... :)
The difference, in my opinion, is that tomato sauce is plain and marina has garlic and herbs (or just garlic and basil, but the point being it's not plain) and most of the time has chunkier tomatoes. You can use tomato sauce to make a marinara sauce.
For me, the definition of "tomato sauce" is different if it's used in a recipe than if it's describing a finished dish.
In a recipe, it usually means the stuff that comes in a can and is next to the tomato paste, crushed tomatoes, diced tomatoes in juice, and other "plain" canned tomato products. It's pretty much a tomato puree, maybe with a little onion powder or other basic flavoring, but it's an ingredient, not a ready-to-use item.
If a menu says something like "pasta with a light tomato sauce", then I translate that to "pasta with a light marinara sauce", meaning that it is a sauce with tomatoes, garlic, basil, etc.
But then if a recipe, perhaps for breadsticks or something, says "serve with a side of tomato sauce", then I would assume that it means a marinara or other spaghetti sauce, not the plain canned tomato sauce.
You have to use context to decide which one is meant.