A useful picture, and well explained IBM document on SOA (Service Oriented Architecture)
….. the fundamental concept is really quite simple – and very powerful. It’s that to meet your present and projected business needs, you can turn your software applications into “building blocks” that you can infinitely rearrange, and usually at great speed. It gives you a new way not only to “reconfigure” your business, but to connect to suppliers, partners and customers.
IBM Global Services: Service-oriented architecture: A practical guide to measuring return on that investment
Bankwatch 
A word of warning. Yes, SOA offers an architectural approach to IT where software applications can be turned into reusable building blocks – BUT this must not be a mechanical process. If the issue of granularity of the building blocks is not thought through, the impact on service levels could be severe.