Put phrases in quotation marks.
Default: AND operators between words.
"james earl ray" retrieves documents in which the words james and earl and ray appear next to each other, in that exact order.
james earl ray (without quotation marks) retrieves documents that contain all 3 words, james and earl and ray, regardless of where the words appear in the document, or in what order.
lincoln AND douglas retrieves only documents that contain both lincoln and douglas.
lincoln OR douglas retrieves documents that contain either lincoln or douglas.