Web of Science will list, when available, two types of keywords: Author Keywords (chosen by the author and submitted with the paper to the journal), and KeyWords Plus (chosen by Web of Science):
SOURCE: Web of Science on the Thomson Reuters Web of Knowledge platform, Copyright ©2014 Thomson Reuters All rights reserved.
Both can be useful - author keywords can give you a sense of how scientists are talking about their fields, and might be useful search terms in other databases. However, they are normally considered uncontrolled vocabulary. This simply means there's no authority looking over the terms and seeing if they are accurate, or eliminating multiple terms for a concept in favor of one preferred term. KeyWords Plus is an example of Web of Science's controlled vocabulary - think of it as a dictionary or thesaurus of terms that Web of Science has chosen to use. Web of Science chooses these terms from titles in the reference list of the document.