The Common Tab is a tab that is available to all PDE and Java launch configurations. It defines general aspects of the launch configuration including storage location, console encoding, etc.
A launch configuration is saved as an XML .launch file on disk. By default, this file is saved in the ${workspace_loc}/.metadata/.plugins/org.eclipse.debug.core/.launches directory, where ${workspace_loc} denotes the workspace location.
If you want to share a launch configuration with team members, you have the option to save it as a Shared file in one of your workspace projects (in order to make it visible for version control).