Keyword searching with one or a few natural language terms is a great way to start your research. It's flexible and forgiving. Database searches for the words anywhere in a book or article record, such as title, author, subject headings/descriptors, abstract, etc. The challenge may be that the search can yield too few, too many, or a certain number of irrelevant, out of context results. However, a good keyword search can also find great results and lead to good subject headings!
Hint: try using the advanced search page, if one exists, in any database or search tool, to help you build your keyword search. Or, try these kinds of approaches in any basic search box too:
"welfare state" and develop* (finds the phrase welfare state and develop, develops, development, developing...)
China and (transit or transportation) (finds china and transit, or china and transportation, or china and both transit and transportation)
Subject headings or pre-defined "controlled vocabulary" terms or phrases are assigned to every item record's subject or descriptor field. Browsing or searching a database this way is a less flexible way to search, since you have to know that vocabulary before you can search on it. However, once you find those important headings (sometimes called descriptors), your results can be extremely relevant. In addition, you can narrow down to sub-headings or sometimes even jump to broader or related subjects to the one you have found.
Examples of subject headings in NUsearch:
mass transit, see: local transit
browsing Local Transit also finds items with other subject headings like Transportation and State and Railroads
Try using the Advanced Search in NUsearch, and search under the Books tab only, to narrow down and gain more control over the initial search.