Tomato sauce and paste may perform similar culinary tasks, but the two are quite different in everything from consistency and ingredients to taste and use.
Tomato sauce is made of tomato purée or fresh tomatoes which are skinned, deseeded, cooked, strained, and mixed with citric acid, salt, water, and seasonings.
Tomato sauce often incorporates additional ingredients like onion and garlic powders, corn syrup, sugar, olive oil, carrot fiber, and even dehydrated bell pepper.
You can use tomato paste as a substitute for tomato sauce by adding water in a one-to-one ratio and mixing in any seasonings to give it a more nuanced flavor.