A service is a process that:
1. is designed to do a small number of things (often just one).
2. has no user interface and is invoked solely via some kind of API.
An application, on the other hand, is pretty much the opposite of that. It has a user interface (even if it’s just a command line) and often performs lots of different tasks. It can also expose an API, but that’s just bonus points.
个人理解，service一般专注做一件事，没有用户界面，并且通过API与application交互。而application会有用户界面，并且通常可以运行很多任务。举个例子，web browser就是application，而web server即为service。
A Kubernetes cluster does not manage a fleet of applications. It manages a cluster of services.A service running in a container managed by Kubernetes is designed to do a very small number of discrete things.
If your services are small and of limited purpose, then they can more easily be scheduled and re-arranged as your load demands. Otherwise, the dependencies become too much to manage and either your scale or your stability suffers.
K8s即是用来管理service的。