First part of question is a pure newbie FAQ to know about, and the simple answer would be it is one of the Clustering type you can adopt within your SQL Server environment. To cover on newbie aspect both Active/Passive & Active/Active configuration...
SQLServer-QA.net - Knowledge Sharing Network (SSQA.net)
Tue, Mar 18 2008
Filed under: Filed under: sql server, performance, security, clustering, 2005, partition, monitoring, active/passive, parallel execution, performance monitoring, active/active, scalability