Service Broker is message queuing for SQL Server. It is used for sending emails and through Database Mails. You can read about SQL SERVER – Difference Between Database Mail and SQLMail here. Service Broker is feature which provides facility to SQL Server to send an asynchronous, transactional message.
In recent user group meeting in my city Ahmedabad, I have found that not every user knows difference between these two features of SQL Server. I do not blame any user for not knowing difference between Database Mail and SQLMail as this is very confusing sometime. I will try to explain this concept here.