What is the difference between questionnaire and inventory?

They are both examples of surveys. Generally a questionnaire asks questions, often requiring relatively simple answers, sometimes using a rating scale, sometimes just true or false. Inventories often list skills or characteristics that may be rated or chosen and often commented upon.

A questionnaire is a form containing a set of questions, especially one addressed to a statistically significant number of subjects as a way of gathering information for a survey.

An inventory is more of a detailed, itemized list, report, or record of characteristics, skills, interests, etc.