At their most general, they are “data processors.” In common parlance, they’re often called “algorithms,” although some folks insist that that is reserved for programs with trivial control flow. For disambiguation and comparison:
A service is an API surface and a contract promising that the surface has certain behaviors; data processing may be part of how the API is implemented. In practice, a service is e.g. an HTTPS endpoint and an OpenAPI specification.
A capability is a copyable token which simultaneously authorizes its holder to perform an action and designates the holder as having the authority to perform that action. This won’t be part of your normal curriculum and training; see this post for an introduction, or this story for motivation.
A controller is a modulator for a (distributed) system. Typically a controller is anything which is actuated by a control loop, although sometimes a controller can sit outside of the system. Common examples include MVC patterns, k8s components, and video-game controllers.
At their most general, they are “data processors.” In common parlance, they’re often called “algorithms,” although some folks insist that that is reserved for programs with trivial control flow. For disambiguation and comparison: