An application based on microservices is composed of small, highly decoupled, mostly autonomous components, that are built to offer a specific (mostly business) functionality. A cloud-native application is composed of multiple microservices that communicated through shared infrastructure, in most cases over HTTP/REST. The microservices architecture provides two major advantages: various components can be developed, deployed, monitored, and troubleshot independently, on a service-by-service basis, rather than dealing with the entire application. The second advantage is that a specific layer can be scaled independently by other layers. The underlying infrastructure can also be provisioned and changed independently and faster.
James Lewis and Martin Fowler definition: Microservice architectural style is an approach to developing a single application as a suite of small services, each running in its own process and communicating with lightweight mechanisms, often an HTTP resource API.