the specified site or domain

site:gov will only retrieve .gov sites

site:cdc.gov will only retrieve cdc.gov pages

the specified filetype

filetype:pdf will only retrieve pdf documents

pages with the exact phrase

"I investigate things" will find pages with those words in that order, pulling up the quote "I investigate things to complete my knowledge" from the show Life. 

pages with either the first or second word/phrase between the parentheses

("mental health" OR "mental hygiene") will find pages that mention either "mental health" or "mental hygiene" or both.

Note: OR must be in caps

research

statistics

public opinion

reviews

pages with the type of information you specify

[ air pollution statistics ] will bring to the top pages with statistical data