I am currently writing the speaker notes for a service-oriented architecture workshop that Microsoft and newtelligence will run later this year. I was just working on the definitions of components and services and I think I found a reasonably short and clear definition for it:

One of the most loaded and least well defined terms in programming is "component". Unfortunately, the same is true for "service". Especially there is confusion about the terms "component" and "services" in the context of SOA.

The term component is a development and deployment concept and refers to some form of compiled code. A component might be a JVM or CLR class, a Java bean or a COM class; in short, a component is any form of a unit of potentially reusable code that can be accessed by name, deployed and activated and can be assembled to build applications. Components are typically implemented using object-oriented programming languages and components can be used to implement services.

A service is a deployment and runtime concept. A service is strictly not a unit of code; it is rather a boundary definition that might be valid for several different concrete implementations. The service definition is deployed along with the components that implement it. The communication to and from a service is governed by data contracts and services policies. From the outside, a service is considered an autonomous unit that is solely responsible for the resource it provides access to. Services are used to compose solutions that may or may not span multiple applications.

Let me repeat the one sentence that made me go “damn, I think now I finally have the topic cornered”:

A service is strictly not a unit of code; it is rather a boundary definition that might be valid for several different concrete implementations.