The 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. These services are built around business capabilities and independently deployable by fully automated deployment machinery. There is a bare minimum of centralized management of these services, which may be written in different programming languages and use different data storage technologies. (-Martin Fowler)
Microservices are both an architecture and an approach to writing software. With microservices, applications are broken down into their smallest components, independent from each other. Instead of a traditional, monolithic, approach to apps, where everything is built into a single piece, microservices are all separated and work together to accomplish the same tasks. Each of these components, or processes, is a microservice. This approach to software development values granularity, being lightweight, and the ability to share similar process across multiple apps. It is a major component of optimizing application development towards a cloud-native model.
Why use a microservice-based infrastructure? The goal is, simply put, to deliver quality software, faster. Using microservices is a means to that end, but there are other considerations too. Breaking your apps into microservices isn’t enough, you've got to manage them, orchestrate them, and deal with the data they create and modify. (adapted from: https://www.redhat.com/en/topics/microservices)
Strangler Pattern:
This is an evolutionary pattern for modernization a legacy application where elements of the application or system are incrementally or iteratively replaced with new components. This is done until the entire application is basically modernized.
Incrementally migrate a legacy system by gradually replacing specific pieces of functionality with new applications and services. As features from the legacy system are replaced, the new system eventually replaces all the old system's features, strangling the old system and allowing you to decommission it.