A utility knife is the tool that takes care of the "in-between" jobs of culinary preparation.
It is between paring and slicing knives, booth in length and shape (typically thin and slender in shape, and between 4"-6") and its function.
Too large for hand-held jobs and too small for medium to large prep jobs. It is, therefore, on the one hand the perfect knife for the non-specialized prep jobs. Conversely, a serrated utility knife is its own specialized knife. For example, on the one hand a straight edged 6" utility knife can be a good knife for making sandwiches because a chef's knife is typically too big for the smaller portions of prep. A serrated utility, on the other hand, is a specialized knife to perform optimal cuts on things like tomato, salami, sausages. Perfect for small jobs that have a tough exterior and softer interior.