What do spaces, projects and boards mean?

Board: This is where you add your content (photos, videos and more). This is the object that moves through the workflow.  Boards can be created within projects, inside a Space or even directly in your workspace, depending on how you want to manage them.

Project: Is simply a collection of Boards. Depending on the need, one may or may not choose to create boards within projects. 

Spaces: A space is a grouping that can be associated with a team within an organisation, a client or any other logical collection that may fit one's need. You can brand spaces with logos, create a talent pool for them and keep projects under relevant spaces. An example of spaces would be an agency creating one each for their clients and managing multiple projects & boards within respective spaces.