Service Broker is a part of the SQL Server that allows secure, transactional messaging between databases that can be within the same server instance or in different ones. The feature first appeared in SQL Server 2005, and has a growing clientele already as SQL Server 2008 nears completion. In a nutshell, Service Broker provides a bi-directional asynchronous message flow called a dialog between a pair of services. Each service is associated with a database and a queue within the database. A queue is a special type of table that allows messages to be received in order. Routes allow services to find each other to create dialogs between them. Asynchronous means that sends are non-blocking, which allows a sending application to proceed with other work while a message is processed concurrently elsewhere. Service Broker commands, such as Send and Receive, are part of TSQL and can be written into procedures. One of the original inspirations for Service Broker was to support Service Oriented Architectures (SOA), but an additional popular use is the "data push" architecture, to allow logging and data warehousing to proceed asynchronously.
This is only a sketch of what Service Broker can do. For more information, here are a couple of books:
SQL Server 2005, Service Broker, by Roger Wolter. Rational Press, 2006. ISBN: 1-932577-27-0 (This is a short, easy-to-read primer)
Pro SQL Server 2005, Service Broker, by Klaus Aschenbrenner. Springer-Verlag, 2007. ISBN: 978-1-59059-842-9 (This goes into greater depth, and also describes C# programming techniques)
Well I guess that about does it for starters. The Service Broker Team will be reading this blog and posting things like coding tips and problem solving help.