Question bank allows a teacher to create, preview, and edit questions in a database of question categories. The teachers can organize the question into categories for easily find the questions and makes the use of random questions and matching question easier.
Here is a short video by Webanywhere, the Moodle partner from UK, about adding different types of questions into question bank on your Moodle site.
You can watch the video directly through this link and embedded below.
What are your best practices about adding questions into question banks, How you manage your question banks on your Moodle site, please share in comments below.