SOA stands for Service Oriented Architecture with SOA pretty much you do things with services.  Wikipedia defines SOA as:

an architectural style that guides all aspects of creating and using business processes, packaged as services, throughout their lifecycle, as well as defining and provisioning the IT infrastructure that allows different applications to exchange data and participate in business processes loosely coupled from the operating systems and programming languages underlying those applications. SOA represents a model in which functionality is decomposed into small, distinct units (services), which can be distributed over a network and can be combined together and reused to create business applications. These services communicate with each other by passing data from one service to another, or by coordinating an activity between two or more services. The concepts of Service Oriented Architecture are often seen as built upon, and the evolution of, the older concepts of distributed computing and modular programming.

A simplistic way I like to "think" of SOA is your main application doesn't actually manually touch any of your data.  Instead you go through say web services to get and do all of your data-centric stuff.  One major reason being I can then build my application or my services in any language I want and just access them through a specific protocol.  To find out more about SOA please read the wikipedia article.  I just wanted to give you a basic concept of SOA.