Pure tomato paste is made by removing the skins and seeds from ripe tomatoes. They are then left to dry in the sun and then pureed into a thick paste. The process of sun-drying gives this tomato paste a richer, deeper and more concentrated tomato flavor. It has an earthy, smoky, and sweet jam-like flavor. This quality tomato paste is somewhat rare and expensive and is usually sold in tubes.
Many commercial tomato pastes are also made by removing the skins and seeds from tomatoes. However, instead of being left to dry in the sun, they are cooked down to form a thick paste. These types of tomato paste can still be quite good, but they do vary in quality and flavor from brand to brand.
Quality commercial tomato pastes should list only tomatoes in the ingredients. Some will use better quality tomatoes than other, so the best way to know is to taste a variety of brands. Many think tomato paste is a cheaper, thicker version of tomato sauce, or that it is simply used as a thickening agent. However, quality tomato paste is primarily used to add rich flavor to a dish.