Is this an interview question?
Anyway, this is more a question for the administration forums.
Furthermore, the question is too vague. There are many types of database mirroring.
But you can start here:
http://msdn.microsoft.com/en-us/library/ms151799.aspx
Need an answer? No, you need a question
My blog at https://sqlkover.com.
MCSE Business Intelligence - Microsoft Data Platform MVP