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Typically, there are a minimum number of services that handle things common to others (such as database access), but each microservice is small and corresponds to a different area of the application.
Furthermore, each one is independent and its code must be able to be deployed without affecting the others. Even each of them can be written in a different programming language, since they only expose the API (a common interface, which does not care about the programming language in which the microservice is programmed below) to the rest of the microservices.
Monolithic approach vs. microservices
In order for you to really understand what we are doing with microservices, I am going to give a very simple example. Imagine that we want to make a web application, Amazon type, but easier. Users enter our website, see the products we have for sale, and when they buy an item, we manage the shipment of the product to their home.