Most search engines employ the principles of Boolean logic in the formulation of search queries. See Boolean Searching on the Internet for detailed information about search strategy. If you take the time to understand the basics of Boolean logic, you will have a better chance of search success.
Search engines tend to have a default Boolean logic.
This means that the space between multiple search terms defaults to either OR logic or AND logic. This has become a de facto standard. It is imperative that you know which logical operator is the default. This will make sure that your search terms are located near each other in the full text document. The closer your terms are placed, the more possibly relevant the document will be.