By default the search engine tries to locate pages which have exact matches for all of the words entered in your search query. If that fails, it then tries to locate pages which contain any words in your search query. If that happens a short message is displayed at the top of the search results indicating this has been done.
In addition, there are several ways to modify the default search behavior.
1.
+ and - qualifiers
If you prepend a word with + that word is required to be on the page.
If you prepend a word with - that word is required to not be on the page. Example: +always -never
2.
* wildcard
If a query word ends with a * all words on a page which start the same way as that query word will match. Example: gift*
3.
? wildcard
If a query word contains a ? any character will match that position. Example: b?g
All of these techniques can be combined: +alway* -ne??r*