Field searching can be particularly useful when you know specific information about an item, like who wrote it (author), or what an item is called (book, DVD, journal or magazine, title). Author searches will return a list of all the books by a certain author, and a title search will give you all items with the search words in the title.
When doing a Quick Search results can be narrowed down by selecting a search criteria (e.g. Title) under Search by. It's best to know what your looking for when searching with a search criteria.
The figure below shows how book information relates to field searching, each book has a description that follow this criteria.