A questionnaire is a form containing a set of questions. After the opening night of your new restaurant, ask your customers to fill out a questionnaire so they can help you know what still needs work.
You write a questionnaire to take a snapshot of what a group of people think. They're not opportunities for deep response, but are good for collecting statistics. If you're hosting a big party, distribute a short questionnaire beforehand to find out how many people are coming, and what music they want the dj to play. If you prefer an older version of the word, say you're giving them a questionary.
