Global Admins are the "Admin" user and members of the Admin group, created when the Vault Professional server is installed.  Global Admins cannot have repository access or project access overridden, but can have Folder Security overridden.  Only Global Admins can see Admin pages under the "Server Settings" category.

Repository Admins have been explicitly given the "Full Admin" access level for a repository, or are in a group which has been given "Full Admin" to a repository.  When Repository Admins connect to the Admin Web Client, they will see the Source Control Repositories section and a list of all repositories for which they are Administrators.  Repository Admins cannot see the Default Repository Settings page.

Project Admins have been explicitly given the "Full Admin" access level for a project, or are in a group which has been given "Full Admin" to a project.  When Project Admins connect to the Admin Web Client, they will see the Item Tracking Projects section and a list of all projects for which they are Administrators.  

It is possible for a user to be both a repository Administrator and a Project Administrator without being a Global Administrator.