SOA is not a technology. It is an architectural approach.
At this level SOA is often associated with web services. However web services only address the low level interactions between services. They do not necessarily tell us about everything that can be done using SOA. Actually SOA is about much more than only web services. SOA is concerned with how an organization implements a business system, the method used and the organization structure.
SOA provides platform independence enabling components to be implemented in different platforms, technologies and languages. This provides great flexibility in assembling new solutions from existing IT components.
SOA components are loosely coupled: Instead of a single rigidly integrated component, there are a number of loosely coupled components which perform the same task. However unlike the tightly structured applications these loosely coupled components can be combined and recombined in a number of ways. This greatly minimizes the impact of change makes the IT infrastructure much more flexible.
That brings us to the classical definition of SOA. If you have understood whatever has been discussed above, this definition will definitely make sense to you.