Database mirroring - Witness only

In SQL Server 2008 R2 Express the only mirroring alternative is "Witness only", however I can't find a good explenation to what this is.

I would like to have a database on a virtual server and then have a local SQL Server that mirrors this database, in case the internet connection goes down, I want to fail-over on the local server.

Any ideas on what SQL Server editions I need to be running on both ends?

For SQL 2008 you can run either Enterprise or Standard for mirroring: http://msdn.microsoft.com/en-us/library/ms188712.aspx

The witness server itself can be Standard, Enterprise, Workgroup, or Express.

The witness server is needed for automatic failover: http://technet.microsoft.com/en-us/library/ms175191.aspx

StackExchange Database Administrators Q#1840, answer score: 7