A project owner is a role within a project which assigns complete control of the project to a user with that role. In the Members secondary tab in the project's primary Info tab, the project owners will have a icon near the right side of their row. Additionally, below their name indicate their role as a "Project Owner".
Project ownership is reserved only for users of internal organizations. Client and vendor members may not be project owners.
Project owners and system Admins are not the same. Project Owners have control over a given project, while Admins manage the system (i.e. all system users, organizations, billing, etc). Though the roles are different, a user may be both a project owner and a system admin.
Depending on a user's project permissions, that user may be able to view all the project members or only a subset of them. Project owners can view all project members.