Let's say you want to search for sites that are about professional football. Do you search for professional or pro football? How about searching for either one:
(professional OR pro) football
The above searches for professional (AND) football or pro (AND) football. Please note that the OR must be all-caps. I don't believe the parentheses are necessary, but I like to put them to disambiguate the logic at least in my own mind (i.e., that the comparison is between professional and pro). They don't hurt anything by being there.