Think of a book report that you have had to write for a class. The class doesn't copy the book and turn it in. Instead each class member decides what is important about the book and writes it down. So if we have a book about social media most people will probably include social media in their description. However; classmate Peter uses the words "online social networks" and Mary and John discuss establishing a personal presence on the web. Computers look for exact matches when processing data. A computer searching through the class book reports for social media would not find either Peter's or Mary's book reviews because they did not use "social media" to describe the book.
Reviews in databases, called abstracts, are like your classmates' book reports. Different reviewers use different words to describe the same thing. If you do not look for the same word that the reviewer used in the abstract to describe something, you won't find all the books and articles written about your topic.
Subject headings or controlled vocabulary are a way to ensure that everyone looking for information about social media finds all the material available. Database planners add a field to the item's description containing uniform, agreed-upon terms describing subject content. So the teacher in our example above would enter all his student book reports into the computer adding the Subject heading: "social media" to each book report for the book. The teacher might also include book reports that the same students had written on The Scarlet Letter and Harry Potter. They would have the subject headings "Literature" and "Fantasy." At the end of the semester, the teacher can use the Find function to find those reports with the words "social media" in the document. Some reports would contain social media in the student description (abstract), some would not, but all the relevant reports would have the subject heading "social media" added by the teacher.
The complete description on an item and any comments added is called a record. In this case each student's book report and the subject heading attached by the professor would be one record. The parts of the record are called fields. Fields in this case would be: author or student who wrote the review, abstract or review, and subject heading assigned by the teacher.
Note: Subject terms are consistent in the database your are in. They may vary database to database, so just be aware of this as you are using them in your research.
I am a student looking for criticism of Little Women by Louisa May Alcott. Which of these subject headings would I use? Hint: Louisa was born in 1832 and died in 1888. Notice that the subject headings are divided into centuries, so while you cannot pick exact date you would pick the appropriate century.