Wikipedia
Microservices are a more concrete and modern interpretation of service-oriented architectures (SOA) used to build distributed software systems. Like in SOA, services in a microservice architecture are processes that communicate with each other over the network in order to fulfill a goal. Also, like in SOA, these services use technology-agnostic protocols. Microservices architectural style is a first realisation of SOA that has happened after the introduction of DevOps and this is becoming the standard for building continuously deployed systems.
In contrast to SOA, microservices gives an answer to the question of how big a service should be and how they should communicate with each other. In a microservices architecture, services should be small and the protocols should be lightweight. The benefit of distributing different responsibilities of the system into different smaller services is that it enhances the cohesion and decreases the coupling. This makes it much easier to change and add functions and qualities to the system anytime. It also allows the architecture of an individual service to emerge through continuous refactoring, hence reduces the need for a big up-front design and allows for releasing the software early and continuously.