A question pool is a set of questions of any types that belongs to the author and others with whom the author shares it.  See What is a Question Pool?

A question pool is available to the author in any worksite in which that person has permission to create assessments, and will show in the list of Question Pools.

You can import a question pool from a saved assessment, or from a file provided by a publisher or exported from another system.  

Note: There is currently no way to export question pools, but the questions can be exported in the form of an assessment.

A question pool is a set of questions, identified by a name, that belongs to you (not the worksite).  You can share a question pool with others, and others can share theirs with you.

Questions pools are set up in advance of an assessment, for convenience.  When you are ready to give your students an assessment (a test or quiz), you can pull questions from your pools and also from pools that have been shared with you.  

Question pools serve as the basis for random-draw questions.  To give each student a different question on the same subject, set up a question pool with several equivalent questions on that subject, and then add a random-draw question using that pool.

Question pools can be subdivided into subpools, and those subpools can be further subdivided, for organization that reflects your teaching methods.

Assessments are subdivided into parts, but may consist of only one part that comprises all the questions.  Parts allow you to create sections of an assessment, each with its own title, questions, question pool draws, attachments (for resources or directions), and question ordering.

When you create a new assessment, a part (i.e., section) called "Default" is created automatically. If you leave it named "Default", that title will not appear on your assessment; to change the part's name, click Edit. You can begin adding questions immediately to "Default", or you can add your own parts.

New parts will be listed in the order you create them. You can re-order parts within an assessment, and edit each part individually.

Your assessment must contain at least one part, but you can remove any of the additional parts you create. You can also remove the "Default" part, as long as you've already created another part to replace it.

For more information on adding parts, see How do I add multiple parts to an assessment?

This explains how to create additional parts to assessment. Parts are often used to set up random question sets that pull questions from question pools.