By definition, microservices, also known as the Microservice Architecture (MSA), is an architectural style and design pattern which says that an application should consist of a collection of loosely-coupled services. This architecture decomposes business domain models into smaller, consistent pieces implemented by services. In other words, each of the services will have its own responsibilities, independent of others, each one of them will provide a specific functionality.
These services should be isolated and autonomous. Yet, they of course need to communicate to provide some piece of business functionality. They usually communicate using REST exposures or by publishing and subscribing events in the publish/subscribe way.
The best way of explaining the reasoning behind the idea of microservices is to compare them with an old, traditional approach...