The default search term operator is AND

You can use brackets, AND OR and NOT to improve your search. For example: C# AND NOT ( VB or SQL )

A query of a single * will return everything, filtered by any filters.

Filter examples:
Search by Author: author:"author name"
Filter by tag: tag:C# or tag:(C++ or Java)
Wildcard search use: "?" or "*" eg. gr?d or gr*