These are the connecting terms -- and, or and not -- that allow you to arrange your search terms with logic and precision. Use and to narrow your search, or to broaden your search, and not to exclude irrelevant records from your search.
Nesting is an extension of Boolean logic. You must always remember to "nest" or group synonymous or related terms linked by the or operator. Normally you use parentheses to nest such terms; alternatively some databases allow you to simply type the terms you want to nest together -- joined by the or operator -- in the same search box.
This is a handy way to cover multiple variations of a search term without having to type in each full-length term; you shorten your term to the first few relevant letters and then add the truncation character. In most databases the truncation character is the asterisk (*) but in some cases it may be a question mark (?) or pound sign (#). (Check the help section of a particular database to find out which character is used.) Example: adolescen* would find "adolescent" or "adolescence".
These terms are sometimes referred to as descriptors, subject headings, or subject terms. They are standardized terms developed as part of the structure of a particular database and used to identify the content of each item in the database in a consistent way. An actual human reviews each item going into a database and assigns the appropriate term or terms. Controlled vocabulary offers another tool for zeroing in on those needles in the database haystack.