The word "and" can give you exclusivity in your search results by combining terms.
The word "or" used in your search is great for words that are synonymns or are used interchangeably.
Finally, the word "not" gives you the greatest control over your results by allowing you to exclude certain terms from your search. For example, the "Gap Year" is a common in Britain when students take time off between high school and college for travel, self-discovery, etc., but you may only be interested in American Gap Year students.
To help with searches put phrases in quotations otherwise databases and search engines will assume that each term is a separate identity, for example:
Truncation is used when a search term has many variations. Place an asterik or question mark at the end of the truncated word when searching. (Note that different databases utilize different truncation tools. See their "Search Tips" for more information.) See examples below: