Subject terms (also sometimes known as descriptors) are words that are used to describe the content or subject of books, articles, and other library materials. They are designed to be efficient and comprehensive ways of describing pieces of information. When you are doing serious research, subject terms should be your best friend--here's how you do it.
When you're in the library catalog looking for books, try doing a "Subject Keyword" search, using the drop-down menu. You'll be searching for books that have your search term in their subject description--it's a good way to keep from getting too many results.
Another thing you can do in the catalog is check the subject terms of the books you find. For example, say you find the perfect book for your research paper, and you want more like it. When you're looking at the book, click on the "Title Info" tab and look at the subjects of the book. You can click on those subjects and do a search for them.
You can also take advantage of subject terms when you are looking for articles in the library databases. Almost all of the articles you find will have a list of associated subject terms--first click on the title of the article, then scroll down to see the subject terms. Just like in the catalog, you can click on them to do a search for articles about those subjects.
You can also narrow your results by subject in databases such as Academic Search Complete--look for a sidebar (like the following picture) after you've done a search. Just click on the subjects to narrow your search if you are getting too many results.
Using subject terms should make your research easier and more effective. Give it a try, and feel free to ask for help from a reference librarian!