To search as a phrase, use "". For example "online social networks"
To connect synonyms, use OR. For example: facebook OR "social media"
use AND to connect concepts. For example: facebook AND "social engagement"
truncate endings of words with an *. For example: engag* finds engage; engages; engagement; engaging
If you only have a single search box and want to do a complicated search string, use (). "social engag*" AND (facebook OR "social media"* ). This is called nesting by librarians.