Different search engines, or search parameter engines, use different parameters, so it would be different dependant on what you where doing.
For example, using Google you can search (repeat exactly as shown):
"password.db"
And it'll search for the words "password" and "db" right next to eachother (most search engines filter out special characters).
You should read up on "Google Hacking"; it's quite a popular subject.
One of my favorites is something similar to:
"gizmo" -site:
UnderGroundNews.com -site:ubbdev.com
And it'll search for the term, excluding the sites you specify.