Phrases in double quotes: For example, "holly dolly" returns very few results as opposed to holly dolly.
Exclusion: Terms can be excluded with -, for example windows -system (note there is no space between "-" and the excluded term).
Wildcard search: Wildcards (symbols representing unknown text) can be prefixed and suffixed, for example, the search *stan will produce results like Kazakhstan and Afghanistan.
For more details, including fuzzy search, Boolean operators, and keywords intitle:, incategory: and prefix:, see Help:Searching.