A SOA can be essentially defined as a collection of services which
communicate with each other. The communication can involve either simple
data passing or it could involve two or more services coordinating some
activity.SOA depends on service-orientation as its main design-principle.

Organization for the Advancement of Structured Information Standards  i.e
OASIS defines SOA as the following:
"A paradigm for organizing and utilizing distributed capabilities that may
be under the control of different ownership domains. It provides a uniform
means to offer, discover, interact with and use capabilities to produce
desired effects consistent with measurable preconditions and expectations."

For a better understanding of SOA we need to understand what Services are:

A service is a function that is well-defined, self-contained, and does not
depend on the context or state of other services.These services do not
embed any calls within there source code but they use defined protocols
which describes how to pass message between them. A service thus presents a
simple interface that abstracts away its underlying complexity, so that
users can access these services without knowledge of the service's platform
implementation.

XML's have been used extensively in SOA to structure data. Web Services
Description Language (WSDL) describes the services whereas SOAP describes
the communication protocol.

SOA's main aim is to bring together different functionalities to form an
application by reusing existing services.But with each service brings with
it some amount of processing overhead and thus we need to take this into
consider before defining the services.