Fabrice has written a very good and informative article about what SOA (Service Oriented Architecture) is all about.
If you're not sure what SOA really means, but you keep hearing this phrase buzzing over your head, this is a very good place to start. I particularly like that Fabrice does not go overboard with drinking the kool-aid and shows a real world implementation and considerations for when and if you should move into a more SOA like implementation. It's not all black and white and I for one don't like technologies and concepts such as this to be as changing as clothing fashion. In fact, I bet we're going to see a title in the near future that says “SOA is the new black“. Ugh.
Fabrice seems to have the right tone of things: not too fast, but just enough to get you going.