Wikipedia :

Applications Architecture

is the science and art of ensuring the suite of applications being used by an organization to create the composite application is scalable, reliable, available and manageable. One not only needs to understand and manage the dynamics of the functionalities the composite application is implementing but also help formulate the deployment strategy and keep an eye out for technological risks that could jeopardize the growth and/or operations of the organization.

Enterprise architecture (EA)

is a rigorous description of the structure of an enterprise, its decomposition into subsystems, the relationships between the subsystems, the relationships with the external environment, the terminology to use, and the guiding principles for the design and evolution of an enterprise This description is comprehensive, including enterprise goals, business functions, business process, roles, organisational structures, business information, software applications and computer systems. Practitioners of EA call themselves “enterprise architects.” An enterprise architect is a person responsible for developing the enterprise architecture and is often called upon to draw conclusions from it. By producing an enterprise architecture, architects are providing a tool for identifying opportunities to improve the enterprise, in a manner that more effectively and efficiently pursues its purpose.

Service-Oriented Architecture (SOA)

 is a flexible set of design principles used during the ases of systems development and integration. A deployed SOA-based architecture will provide a loosely-integrated suite of services that can be used within multiple business domains. SOA also generally provides a way for consumers of services, such as web-based applications, to be aware of available SOA-based services. For example, several disparate departments within a company may develop and deploy SOA services in different implementation languages; their respective clients will benefit from a well understood, well defined interface to access them. XML is commonly used for interfacing with SOA services, though this is not required. SOA defines how to integrate widely disparate applications for a world that is Web based and uses multiple implementation platforms. Rather than defining an API, SOA defines the interface in terms of protocols and functionality. An endpoint is the entry point for such an SOA implementation. Service-orientation requires loose coupling of services with operating systems, and other technologies that underlie applications. SOA separates functions into distinct units, or services[1], which developers make accessible over a network in order to allow users to combine and reuse them in the production of applications. These services and their corresponding consumers communicate with each other by passing data in a well-defined, shared format, or by coordinating an activity between two or more services