Service Broker is a unique feature in Microsoft SQL Server 2005 which is used to by internal or external processes to send and receive guaranteed, asynchronous messages by using extensions to Transact-SQL Data Manipulation Language (DML).
It's a platform for building a new class of loosely coupled services for database applications. Service Broker not only brings asynchronous, queued messaging to database applications but significantly expands the state of the art for reliable messaging.
The key components of SQL Server Service broker are
.Queue stores the messages for a particular service.
.Dialog is a conversation between two services.
.Conversation group is a group of related conversations. Every conversation belongs to exactly one conversation group.
.Activation specifies a stored procedure that will handle messages destined for a particular service.