To broaden search, use OR.
This will retrieve results that have either of your search terms.
Ex. Classical OR Romantic
To narrow search, use AND.
This will retrieve results that have both of your search terms.
Ex. Picasso AND cubism
Use " " to search for terms together, and not separately.
Ex. "Roman Catholic"
Use * to search for multiple endings to words all at once.
Ex. searching theat* will retrieve theatre, theater, theatrical, etc.