SOA, at its essence, is an architectural pattern that actually predates the SOA buzzword. It’s about breaking an architecture down to a functional primitive, understanding the information and behaviors, and building it up again using service interfaces that are abstracted into a configuration layer to create and, more importantly, re-create business solutions. Very simple point.
Cloud computing is a type of solution, a way of creating a system in which some or all of its IT resources exist within some third-party cloud computing resource, such as Amazon EC2 or Force.com. Thus, cloud computing is something that can involve part of or all of an architecture. The core difference is that the system is extended to resources that you don’t own or host locally.
Putting this more simply, SOA is all about the process of defining an IT solution or architecture, while cloud computing is an architectural alternative. Thus, SOA can’t be replaced by cloud computing. In fact, most cloud computing solutions are going to be defined through SOA. They don’t compete — they are complementary notions.
节选自：http://www.gcn.com/Articles/2009/03/09/Guest-commentary-SOA-cloud.aspx
Definitions:
Software as a Service: Software provides an application on-demand. There is no implied language, development methodology, or tool specifically attributed to SaaS. Some development methods may be more appropriate (such as Java and C#) since SaaS applications often provide the user interface a browser .
Service Oriented Architecture: (SOA) provides methods for systems development and integration where systems group functionality around business processes and package these as interoperable services. A SOA infrastructure allows different applications to exchange data with one another as they participate in business processes. Some organizations offer software as a service running on the organization’s private infrastructure as well.
Cloud Computing: Cloud computing is Internet (cloud) based use of computer technology where dynamically scalable resources are provided as a service over the Internet. Users need not have knowledge of, expertise in, or control over the technology infrastructure (the Infrastructure as a Service cloud) that supports them…virtualized. Some call this “IT Infrastructure as a Service. ” Some venders refer to the “private cloud,” which is essentially virtualized local servers. Gotta love the buzzwords.
SaaS applications may use the cloud but they are not the cloud.
SOA architectures may or may not be delivered via SaaS but they are not generically SaaS.
Cloud applications may or may not be delivered as SaaS