Keywords are the words you use to describe the main concept of your topic. They are also referred to as "natural language." When you search in a database using keywords, the database typically searches author, title, subject. Sometimes it also looks for these keywords within the article or book, or video, etc.
A keyword search can be the first step on the way to finding subject headings that are appropriate to your topic and using them to get more relevant results.
Subject Headings or descriptors are pre-determined words that describe the main concept of an article. Many databases use subject headings that are unique to that particular database, e.g. ERIC, PsycINFO or CINAHL or MEDLINE. You are using "controlled vocabulary" when you search using subject headings. Unlike keyword searching, a subject search looks in only one field of each record - the subject field.
You are likely to get relevant results using controlled vocabulary; however, it is not always possible to search using controlled vocabulary. Perhaps your topic is very new. Perhaps the database does not have an online thesaurus to browse for the subject terms you need for your topic. In such cases: